diff --git a/lib/core.js b/lib/core.js
index 5dda9cd089a..0f6bc6c049b 100644
--- a/lib/core.js
+++ b/lib/core.js
@@ -699,7 +699,7 @@ declare var Math: {
  */
 declare class $ReadOnlyArray<+T> {
     @@iterator(): Iterator<T>;
-        /**
+    /**
      * Returns a string representation of an array. The elements are converted to string using their toLocalString methods.
      */
     toLocaleString(): string;
@@ -1742,24 +1742,142 @@ type IteratorResult<+Yield,+Return> =
     ...
 };
 
-interface $Iterator<+Yield,+Return,-Next> extends $Iterable<Yield,Return,Next> {
-    @@iterator(): $Iterator<Yield,Return,Next>;
+/**
+ * The iterator protocol expected by built-ins like Iterator.from().
+ * You can implement this yourself.
+ */
+interface $IteratorProtocol<+Yield,+Return=void,-Next=void> {
     next(value?: Next): IteratorResult<Yield,Return>;
 }
-type Iterator<+T> = $Iterator<T,void,void>;
 
+/**
+ * The built-in Iterator abstract base class. Iterators for Arrays, Strings, and Generators all inherit from this class.
+ * Extend this class to implement custom iterators that support all iterator helper methods.
+ * Note that you can use `Iterator.from()` to get an iterator helper wrapping any object that implements
+ * the iterator protocol.
+ */
+declare class Iterator<+Yield,+Return=void,-Next=void> implements $IteratorProtocol<Yield,Return,Next>, $Iterable<Yield,Return,Next> {
+    @@iterator(): Iterator<Yield,Return,Next>;
+    next(value?: Next): IteratorResult<Yield,Return>;
+    /**
+     * Returns a new iterator that yields that values returned by calling the argument callback on each value yielded by this iterator.
+     * @param callbackfn A function that accepts up to two arguments: the yielded value and its index. The map method calls the callbackfn function once per iteration.
+     */
+    map<U>(callbackfn: (value: Yield, index: number) => U): Iterator<U, void, mixed>;
+    /**
+     * Returns a new iterator that yields only those elements of the iterator for which the provided predicate returns a truthy value.
+     * @param callbackfn A function that accepts up to two arguments: the yielded value and its index. The filter method calls the predicate function once per iteration.
+     */
+    filter(callbackfn: typeof Boolean): Iterator<$NonMaybeType<Yield>, void, mixed>;
+    /**
+     * Returns a new iterator that yields only those elements of the iterator for which the provided predicate returns a truthy value.
+     * @param callbackfn A function that accepts up to two arguments: the yielded value and its index. The filter method calls the predicate function once per iteration.
+     */
+    filter<Refined: Yield>(callbackfn: (value: Yield, index: number) => implies value is Refined): Iterator<Refined, void, mixed>;
+    /**
+     * Returns a new iterator that yields only those elements of the iterator for which the provided predicate returns a truthy value.
+     * @param callbackfn A function that accepts up to two arguments: the yielded value and its index. The filter method calls the predicate function once per iteration.
+     */
+    filter(callbackfn: (value: Yield, index: number) => mixed): Iterator<Yield, void, mixed>;
+    /**
+     * Returns a new iterator that yields up to the given number of elements in this iterator and then terminates.
+     * @param limit The maximum number of values to yield.
+     */
+    take(limit: number): Iterator<Yield, void, mixed>;
+    /**
+     * Returns a new iterator that yields values from this iterator after skipping the provided count. If the this iterator has fewer than limit elements, the new iterator will be completed the first time next() is called.
+     * @param count The number of values to drop.
+     */
+    drop(count: number): Iterator<Yield, void, mixed>;
+    /**
+     * Returns a new iterator that takes each value yielded by this iterator, runs it through the argument mapping function to get another iterable, and yields each value returned by these mapped iterables.
+     * @param callbackfn A function that accepts up to two arguments: the yielded value and its index. The map method calls the callbackfn function once per iteration. The callback must return an iterable.
+     */
+    flatMap<U>(callbackfn: (value: Yield, index: number) => $IteratorProtocol<U, mixed, void> | $Iterable<U, mixed, void>): Iterator<U, void, mixed>;
+    /**
+     * Calls the argument "reducer" callback function on each value produced by this iterator, passing in the return value from the calculation on the preceding iteration. The final result of running the reducer across all elements is a single value.
+     * Exhausts this iterator. Will hang if it's infinite.
+     * @param callbackfn A function that accepts up to three arguments: the accumulated value, the current value, and the current index.
+     * @param initialValue If initialValue is specified, it is used as the initial value to start
+     * the accumulation. If omitted, the first call of the callback gets the first two iterated values.
+     */
+    reduce<U>(
+      callbackfn: (previousValue: Yield | U, currentValue: Yield, index: number) => U,
+    ): Yield | U;
+    /**
+     * Calls the argument "reducer" callback function on each value produced by this iterator, passing in the return value from the calculation on the preceding iteration. The final result of running the reducer across all elements is a single value.
+     * Exhausts this iterator. Will hang if it's infinite.
+     * @param callbackfn A function that accepts up to three arguments: the accumulated value, the current value, and the current index.
+     * @param initialValue If initialValue is specified, it is used as the initial value to start
+     * the accumulation. If omitted, the first call of the callback gets the first two iterated values.
+     */
+    reduce<U>(
+      callbackfn: (previousValue: U, currentValue: Yield, index: number) => U,
+      initialValue: U
+    ): U;
+    /**
+     * Creates a new array from the values yielded by this iterator.
+     * Exhausts this iterator. Will hang if it's infinite.
+     */
+    toArray(): Array<Yield>;
+    /**
+     * Calls the argument function for each value yielded by the iterator. Any returned values are ignored.
+     * Exhausts this iterator. Will hang if it's infinite.
+     * @param callbackfn  A function that accepts up to two arguments: the yielded value and its index.
+     */
+    forEach(callbackfn: (value: Yield, index: number) => mixed): void;
+    /**
+     * Calls the argument function for each value yielded by the iterator.
+     * Returns true if the predicate function ever returns a truthy value.
+     * Exhausts this iterator. Will hang if it's infinite.
+     * @param callbackfn  A function that accepts up to two arguments: the yielded value and its index.
+     */
+    some(callbackfn: (value: Yield, index: number) => mixed): boolean;
+    /**
+     * Calls the argument function for each value yielded by the iterator.
+     * Returns true if the predicate function returns a truthy value for all iterated values.
+     * Exhausts this iterator. Will hang if it's infinite.
+     * @param callbackfn  A function that accepts up to two arguments: the yielded value and its index.
+     */
+    every(callbackfn: (value: Yield, index: number) => mixed): boolean;
+    /**
+     * Calls the argument function for values yielded by the iterator until one returns a truthy value.
+     * Returns the first value that produced a truthy predicate.
+     * Returns undefined if the iterator completes without a match.
+     * Iterates this iterator. Will hang if it's infinite and has no matches.
+     * @param callbackfn  A function that accepts up to two arguments: the yielded value and its index.
+     */
+    find<Refined: Yield>(predicate: (value: Yield, index: number) => implies value is Refined): Refined | void;
+    find(callbackfn: (value: Yield, index: number) => mixed): Yield | void;
+    /**
+     * This method allows wrapping objects that implement the iterator protocol in an Iterator to get the built-in helpers.
+     * @returns The argument object if it's already an Iterator subclass, or a wrapping Iterator if the argument objects implements the iterable or iterator protocols.
+     */
+    static from<+Yield,+Return,-Next>(source: $IteratorProtocol<Yield,Return,Next> | $Iterable<Yield,Return,Next>): Iterator<Yield,Return,Next>;
+}
+
+type $Iterator<+Yield,+Return,-Next> = Iterator<Yield,Return,Next>;
+
+/**
+ * The iterable protocol expected by built-ins like Array.from().
+ * You can implement this yourself.
+ */
 interface $Iterable<+Yield,+Return,-Next> {
-    @@iterator(): $Iterator<Yield,Return,Next>;
+    @@iterator(): $IteratorProtocol<Yield,Return,Next>;
 }
 type Iterable<+T> = $Iterable<T,void,void>;
 
-interface Generator<+Yield,+Return,-Next> {
-    @@iterator(): $Iterator<Yield,Return,Next>;
-    next(value?: Next): IteratorResult<Yield,Return>;
+// The generator class is not exposed in the global namespace.
+declare class $Generator<+Yield,+Return,-Next> extends Iterator<Yield,Return,Next> {
     return<R>(value: R): IteratorResult<Yield,R|Return>;
     throw(error?: any): IteratorResult<Yield,Return>;
 }
 
+/**
+ * Built-in Generator objects returned by generator functions.
+ */
+type Generator<+Yield,+Return,-Next> = $Generator<Yield,Return,Next>;
+
 declare function $iterate<T>(p: Iterable<T>): T;
 
 /* Async Iterable/Iterator/Generator */
diff --git a/tests/async/async.exp b/tests/async/async.exp
index d48fc01c48a..82aa49909e5 100644
--- a/tests/async/async.exp
+++ b/tests/async/async.exp
@@ -10,8 +10,8 @@ References:
    async.js:9:30
       9| async function f1(): Promise<boolean> {
                                       ^^^^^^^ [2]
-   <BUILTINS>/core.js:1984:24
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:24
+   2102| declare class Promise<+R = mixed> {
                                 ^ [3]
 
 
@@ -31,8 +31,8 @@ References:
    async.js:28:48
      28| async function f4(p: Promise<number>): Promise<boolean> {
                                                         ^^^^^^^ [2]
-   <BUILTINS>/core.js:1984:24
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:24
+   2102| declare class Promise<+R = mixed> {
                                 ^ [3]
 
 
@@ -99,8 +99,8 @@ undefined in type argument `R` [2]. [incompatible-return]
                      ^^^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:1984:24
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:24
+   2102| declare class Promise<+R = mixed> {
                                 ^ [2]
 
 
@@ -142,8 +142,8 @@ References:
    async_return_void.js:1:32
       1| async function foo1(): Promise<string> {
                                         ^^^^^^ [2]
-   <BUILTINS>/core.js:1984:24
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:24
+   2102| declare class Promise<+R = mixed> {
                                 ^ [3]
 
 
@@ -160,8 +160,8 @@ References:
    async_return_void.js:5:32
       5| async function foo2(): Promise<string> {
                                         ^^^^^^ [2]
-   <BUILTINS>/core.js:1984:24
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:24
+   2102| declare class Promise<+R = mixed> {
                                 ^ [3]
 
 
@@ -181,8 +181,8 @@ References:
    async_return_void.js:9:32
       9| async function foo3(): Promise<string> {
                                         ^^^^^^ [2]
-   <BUILTINS>/core.js:1984:24
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:24
+   2102| declare class Promise<+R = mixed> {
                                 ^ [3]
 
 
@@ -220,11 +220,11 @@ string [2] in type argument `Yield` [3]. [incompatible-return]
                                                      ^^^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:2047:58
-   2047|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
+   <BUILTINS>/core.js:2165:58
+   2165|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
                                                                   ^^^^^^^^^^^^^ [2]
-   <BUILTINS>/core.js:1778:27
-   1778| interface AsyncGenerator<+Yield,+Return,-Next> {
+   <BUILTINS>/core.js:1896:27
+   1896| interface AsyncGenerator<+Yield,+Return,-Next> {
                                    ^^^^^ [3]
 
 
@@ -237,14 +237,14 @@ string [1] is incompatible with number [2] in type argument `Yield` [3]. [incomp
                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 References:
-   <BUILTINS>/core.js:2047:58
-   2047|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
+   <BUILTINS>/core.js:2165:58
+   2165|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
                                                                   ^^^^^^^^^^^^^ [1]
    generator.js:21:45
      21| async function *genError1(): AsyncGenerator<number, void, void> {
                                                      ^^^^^^ [2]
-   <BUILTINS>/core.js:1778:27
-   1778| interface AsyncGenerator<+Yield,+Return,-Next> {
+   <BUILTINS>/core.js:1896:27
+   1896| interface AsyncGenerator<+Yield,+Return,-Next> {
                                    ^^^^^ [3]
 
 
@@ -258,8 +258,8 @@ Cannot yield `1` because number [1], a primitive, cannot be used as a subtype of
                     ^ [1]
 
 References:
-   <BUILTINS>/core.js:1789:7
-   1789|     : $Iterable<Yield, Return, Next>;
+   <BUILTINS>/core.js:1907:7
+   1907|     : $Iterable<Yield, Return, Next>;
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ [2]
 
 
diff --git a/tests/autocomplete/autocomplete.exp b/tests/autocomplete/autocomplete.exp
index e9c12f61b85..54b1e70a852 100644
--- a/tests/autocomplete/autocomplete.exp
+++ b/tests/autocomplete/autocomplete.exp
@@ -1311,6 +1311,7 @@ Flags: --pretty
     {"name":"Intl$PluralRules","type":"typeof Intl$PluralRules"},
     {"name":"isFinite","type":"(number: mixed) => boolean"},
     {"name":"isNaN","type":"(number: mixed) => boolean"},
+    {"name":"Iterator","type":"typeof Iterator"},
     {
       "name":"JSON",
       "type":"{|+parse: (text: string, reviver?: (key: any, value: any) => any) => any, +stringify: ((value: null | string | number | boolean | interface {} | $ReadOnlyArray<mixed>, replacer?: ?((key: string, value: any) => any) | Array<any>, space?: string | number) => string) & ((value: mixed, replacer?: ?((key: string, value: any) => any) | Array<any>, space?: string | number) => string | void)|}"
diff --git a/tests/babel_loose_array_spread/babel_loose_array_spread.exp b/tests/babel_loose_array_spread/babel_loose_array_spread.exp
index 179e4602264..b3b95a786c9 100644
--- a/tests/babel_loose_array_spread/babel_loose_array_spread.exp
+++ b/tests/babel_loose_array_spread/babel_loose_array_spread.exp
@@ -16,6 +16,24 @@ References:
                                     ^^^^^^^^^^^^^^^^^ [2]
 
 
+Error --------------------------------------------------------------------------------------------------- apply.js:11:15
+
+Cannot call `f.apply` because `$IteratorProtocol` [1] is incompatible with `Iterator` [2] in the return value of
+property `@@iterator` of type argument `A`. [incompatible-call]
+
+   apply.js:11:15
+     11| f.apply(null, it); // Error
+                       ^^
+
+References:
+   <BUILTINS>/core.js:1866:19
+   1866|     @@iterator(): $IteratorProtocol<Yield,Return,Next>;
+                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ [1]
+   <BUILTINS>/core.js:1149:17
+   1149|   @@iterator(): Iterator<T>;
+                         ^^^^^^^^^^^ [2]
+
+
 Error ------------------------------------------------------------------------------------------------- iterables.js:2:5
 
 `$Iterable` [1] is incompatible with `$ReadOnlyArray` [2]. [incompatible-type]
@@ -187,4 +205,4 @@ References:
 
 
 
-Found 11 errors
+Found 12 errors
diff --git a/tests/badly_positioned_func_proto/badly_positioned_func_proto.exp b/tests/badly_positioned_func_proto/badly_positioned_func_proto.exp
index d0b01684b64..e61be9d9e72 100644
--- a/tests/badly_positioned_func_proto/badly_positioned_func_proto.exp
+++ b/tests/badly_positioned_func_proto/badly_positioned_func_proto.exp
@@ -8,8 +8,8 @@ it into an object and attempt to use it as a subtype of an interface. [incompati
                       ^ [1]
 
 References:
-   <BUILTINS>/core.js:1751:11
-   1751| interface $Iterable<+Yield,+Return,-Next> {
+   <BUILTINS>/core.js:1865:11
+   1865| interface $Iterable<+Yield,+Return,-Next> {
                    ^^^^^^^^^ [2]
 
 
@@ -23,8 +23,8 @@ it into an object and attempt to use it as a subtype of an interface. [incompati
                         ^ [1]
 
 References:
-   <BUILTINS>/core.js:1751:11
-   1751| interface $Iterable<+Yield,+Return,-Next> {
+   <BUILTINS>/core.js:1865:11
+   1865| interface $Iterable<+Yield,+Return,-Next> {
                    ^^^^^^^^^ [2]
 
 
diff --git a/tests/bigint/bigint.exp b/tests/bigint/bigint.exp
index 7ed51afbc6f..a726bafe955 100644
--- a/tests/bigint/bigint.exp
+++ b/tests/bigint/bigint.exp
@@ -30,23 +30,23 @@ Cannot call `BigInt` with `null` bound to `value` because: [incompatible-call]
                 ^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:2774:18
-   2774|   static (value: boolean | string | number | bigint | interface {} | $ReadOnlyArray<mixed>): bigint;
+   <BUILTINS>/core.js:2892:18
+   2892|   static (value: boolean | string | number | bigint | interface {} | $ReadOnlyArray<mixed>): bigint;
                           ^^^^^^^ [2]
-   <BUILTINS>/core.js:2774:28
-   2774|   static (value: boolean | string | number | bigint | interface {} | $ReadOnlyArray<mixed>): bigint;
+   <BUILTINS>/core.js:2892:28
+   2892|   static (value: boolean | string | number | bigint | interface {} | $ReadOnlyArray<mixed>): bigint;
                                     ^^^^^^ [3]
-   <BUILTINS>/core.js:2774:37
-   2774|   static (value: boolean | string | number | bigint | interface {} | $ReadOnlyArray<mixed>): bigint;
+   <BUILTINS>/core.js:2892:37
+   2892|   static (value: boolean | string | number | bigint | interface {} | $ReadOnlyArray<mixed>): bigint;
                                              ^^^^^^ [4]
-   <BUILTINS>/core.js:2774:46
-   2774|   static (value: boolean | string | number | bigint | interface {} | $ReadOnlyArray<mixed>): bigint;
+   <BUILTINS>/core.js:2892:46
+   2892|   static (value: boolean | string | number | bigint | interface {} | $ReadOnlyArray<mixed>): bigint;
                                                       ^^^^^^ [5]
-   <BUILTINS>/core.js:2774:55
-   2774|   static (value: boolean | string | number | bigint | interface {} | $ReadOnlyArray<mixed>): bigint;
+   <BUILTINS>/core.js:2892:55
+   2892|   static (value: boolean | string | number | bigint | interface {} | $ReadOnlyArray<mixed>): bigint;
                                                                ^^^^^^^^^^^^ [6]
-   <BUILTINS>/core.js:2774:70
-   2774|   static (value: boolean | string | number | bigint | interface {} | $ReadOnlyArray<mixed>): bigint;
+   <BUILTINS>/core.js:2892:70
+   2892|   static (value: boolean | string | number | bigint | interface {} | $ReadOnlyArray<mixed>): bigint;
                                                                               ^^^^^^^^^^^^^^^^^^^^^ [7]
 
 
diff --git a/tests/component_type/component_type.exp b/tests/component_type/component_type.exp
index 631abb32eb1..849b8b08fa5 100644
--- a/tests/component_type/component_type.exp
+++ b/tests/component_type/component_type.exp
@@ -573,8 +573,8 @@ References:
    everything_implicit_instantiation.js:6:46
       6| declare component B(ref: React.RefSetter<Set<string>>, foo: string, bar: number) renders? A;
                                                       ^^^^^^ [2]
-   <BUILTINS>/core.js:1942:19
-   1942| declare class Set<T> extends $ReadOnlySet<T> {
+   <BUILTINS>/core.js:2060:19
+   2060| declare class Set<T> extends $ReadOnlySet<T> {
                            ^ [3]
    <BUILTINS>/react.js:235:19
     235|   type RefSetter<-T> = React$RefSetter<T>;
@@ -929,8 +929,8 @@ References:
    props_implicit_instantiation.js:7:46
       7| declare component A(ref: React.RefSetter<Set<string>>, foo: string, bar: number);
                                                       ^^^^^^ [2]
-   <BUILTINS>/core.js:1942:19
-   1942| declare class Set<T> extends $ReadOnlySet<T> {
+   <BUILTINS>/core.js:2060:19
+   2060| declare class Set<T> extends $ReadOnlySet<T> {
                            ^ [3]
    <BUILTINS>/react.js:235:19
     235|   type RefSetter<-T> = React$RefSetter<T>;
diff --git a/tests/contextual_typing/contextual_typing.exp b/tests/contextual_typing/contextual_typing.exp
index cee78ba08ce..01c84f92382 100644
--- a/tests/contextual_typing/contextual_typing.exp
+++ b/tests/contextual_typing/contextual_typing.exp
@@ -22,8 +22,8 @@ expression to your expected type. [underconstrained-implicit-instantiation]
                             ^^^
 
 References:
-   <BUILTINS>/core.js:1942:19
-   1942| declare class Set<T> extends $ReadOnlySet<T> {
+   <BUILTINS>/core.js:2060:19
+   2060| declare class Set<T> extends $ReadOnlySet<T> {
                            ^ [1]
    arr_rest.js:9:16
       9| const [...y] = new Set() // still error
@@ -322,8 +322,8 @@ Property `@@iterator` is missing in function [1] but exists in `$Iterable` [2].
                 ^^^^^^^^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:1751:11
-   1751| interface $Iterable<+Yield,+Return,-Next> {
+   <BUILTINS>/core.js:1865:11
+   1865| interface $Iterable<+Yield,+Return,-Next> {
                    ^^^^^^^^^ [2]
 
 
@@ -461,8 +461,8 @@ References:
    implicit_instantiation.js:147:11
     147|   (a: Set<empty>); // error Set<string> ~> Set<empty>
                    ^^^^^ [2]
-   <BUILTINS>/core.js:1942:19
-   1942| declare class Set<T> extends $ReadOnlySet<T> {
+   <BUILTINS>/core.js:2060:19
+   2060| declare class Set<T> extends $ReadOnlySet<T> {
                            ^ [3]
 
 
@@ -481,8 +481,8 @@ References:
    implicit_instantiation.js:148:11
     148|   (b: Set<empty>); // error Set<string> ~> Set<empty>
                    ^^^^^ [2]
-   <BUILTINS>/core.js:1942:19
-   1942| declare class Set<T> extends $ReadOnlySet<T> {
+   <BUILTINS>/core.js:2060:19
+   2060| declare class Set<T> extends $ReadOnlySet<T> {
                            ^ [3]
 
 
@@ -723,7 +723,7 @@ Cannot determine type of empty array literal. Please provide an annotation. [mis
 
 Error ----------------------------------------------------------------------------------------------------- lits.js:52:3
 
-Cannot cast `s2.values()` to `Iterator` because number [1] is incompatible with string [2] in type argument `T` [3].
+Cannot cast `s2.values()` to `Iterator` because number [1] is incompatible with string [2] in type argument `Yield` [3].
 [incompatible-cast]
 
    lits.js:52:3
@@ -737,9 +737,9 @@ References:
    lits.js:52:27
      52|   s2.values() as Iterator<string>; // error number ~> string
                                    ^^^^^^ [2]
-   <BUILTINS>/core.js:1749:16
-   1749| type Iterator<+T> = $Iterator<T,void,void>;
-                        ^ [3]
+   <BUILTINS>/core.js:1759:25
+   1759| declare class Iterator<+Yield,+Return=void,-Next=void> implements $IteratorProtocol<Yield,Return,Next>, $Iterable<Yield,Return,Next> {
+                                 ^^^^^ [3]
 
 
 Error ------------------------------------------------------------------------------------------ misplaced_error.js:9:31
diff --git a/tests/core_tests/core_tests.exp b/tests/core_tests/core_tests.exp
index 1fe6ecf24e0..8e78a374e01 100644
--- a/tests/core_tests/core_tests.exp
+++ b/tests/core_tests/core_tests.exp
@@ -75,12 +75,12 @@ References:
    map.js:21:22
      21|     let x = new Map(['foo', 123]); // error
                               ^^^^^ [1]
-   <BUILTINS>/core.js:1815:38
-   1815|     constructor(iterable?: ?Iterable<[K, V]>): void;
+   <BUILTINS>/core.js:1933:38
+   1933|     constructor(iterable?: ?Iterable<[K, V]>): void;
                                               ^^^^^^ [2]
-   <BUILTINS>/core.js:1745:22
-   1745| interface $Iterator<+Yield,+Return,-Next> extends $Iterable<Yield,Return,Next> {
-                              ^^^^^ [3]
+   <BUILTINS>/core.js:1749:30
+   1749| interface $IteratorProtocol<+Yield,+Return=void,-Next=void> {
+                                      ^^^^^ [3]
 
 
 Error ----------------------------------------------------------------------------------------------------- map.js:21:21
@@ -96,12 +96,12 @@ References:
    map.js:21:29
      21|     let x = new Map(['foo', 123]); // error
                                      ^^^ [1]
-   <BUILTINS>/core.js:1815:38
-   1815|     constructor(iterable?: ?Iterable<[K, V]>): void;
+   <BUILTINS>/core.js:1933:38
+   1933|     constructor(iterable?: ?Iterable<[K, V]>): void;
                                               ^^^^^^ [2]
-   <BUILTINS>/core.js:1745:22
-   1745| interface $Iterator<+Yield,+Return,-Next> extends $Iterable<Yield,Return,Next> {
-                              ^^^^^ [3]
+   <BUILTINS>/core.js:1749:30
+   1749| interface $IteratorProtocol<+Yield,+Return=void,-Next=void> {
+                                      ^^^^^ [3]
 
 
 Error ----------------------------------------------------------------------------------------------------- map.js:22:42
@@ -120,9 +120,9 @@ References:
    map.js:22:16
      22|     let y: Map<number, string> = new Map([['foo', 123]]); // error
                         ^^^^^^ [2]
-   <BUILTINS>/core.js:1745:22
-   1745| interface $Iterator<+Yield,+Return,-Next> extends $Iterable<Yield,Return,Next> {
-                              ^^^^^ [3]
+   <BUILTINS>/core.js:1749:30
+   1749| interface $IteratorProtocol<+Yield,+Return=void,-Next=void> {
+                                      ^^^^^ [3]
 
 
 Error ----------------------------------------------------------------------------------------------------- map.js:22:42
@@ -141,9 +141,9 @@ References:
    map.js:22:24
      22|     let y: Map<number, string> = new Map([['foo', 123]]); // error
                                 ^^^^^^ [2]
-   <BUILTINS>/core.js:1745:22
-   1745| interface $Iterator<+Yield,+Return,-Next> extends $Iterable<Yield,Return,Next> {
-                              ^^^^^ [3]
+   <BUILTINS>/core.js:1749:30
+   1749| interface $IteratorProtocol<+Yield,+Return=void,-Next=void> {
+                                      ^^^^^ [3]
 
 
 Error ------------------------------------------------------------------------------------------------------ map.js:27:6
@@ -155,8 +155,8 @@ Cannot cast `x.get(...)` to boolean because undefined [1] is incompatible with b
               ^^^^^^^^^^^^
 
 References:
-   <BUILTINS>/core.js:1823:22
-   1823|     get(key: K): V | void;
+   <BUILTINS>/core.js:1941:22
+   1941|     get(key: K): V | void;
                               ^^^^ [1]
    map.js:27:20
      27|     (x.get('foo'): boolean); // error, string | void
@@ -206,11 +206,11 @@ Cannot call `WeakRef` because in type argument `T`: [incompatible-call]
                                ^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:1840:28
-   1840| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
+   <BUILTINS>/core.js:1958:28
+   1958| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
                                     ^^^^^^^^^^^^ [2]
-   <BUILTINS>/core.js:1840:43
-   1840| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
+   <BUILTINS>/core.js:1958:43
+   1958| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
                                                    ^^^^^^^^^^^^^^^^^^^^^ [3]
 
 
@@ -226,11 +226,11 @@ Cannot call `WeakRef` because in type argument `T`: [incompatible-call]
                                ^^^ [1]
 
 References:
-   <BUILTINS>/core.js:1840:28
-   1840| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
+   <BUILTINS>/core.js:1958:28
+   1958| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
                                     ^^^^^^^^^^^^ [2]
-   <BUILTINS>/core.js:1840:43
-   1840| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
+   <BUILTINS>/core.js:1958:43
+   1958| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
                                                    ^^^^^^^^^^^^^^^^^^^^^ [3]
 
 
@@ -246,11 +246,11 @@ Cannot call `WeakRef` because in type argument `T`: [incompatible-call]
                                ^^^^^^^^^^^^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:1840:28
-   1840| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
+   <BUILTINS>/core.js:1958:28
+   1958| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
                                     ^^^^^^^^^^^^ [2]
-   <BUILTINS>/core.js:1840:43
-   1840| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
+   <BUILTINS>/core.js:1958:43
+   1958| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
                                                    ^^^^^^^^^^^^^^^^^^^^^ [3]
 
 
@@ -266,11 +266,11 @@ Cannot call `WeakSet` because in type argument `T`: [incompatible-call]
                                 ^ [1]
 
 References:
-   <BUILTINS>/core.js:1840:28
-   1840| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
+   <BUILTINS>/core.js:1958:28
+   1958| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
                                     ^^^^^^^^^^^^ [2]
-   <BUILTINS>/core.js:1840:43
-   1840| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
+   <BUILTINS>/core.js:1958:43
+   1958| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
                                                    ^^^^^^^^^^^^^^^^^^^^^ [3]
 
 
@@ -286,11 +286,11 @@ Cannot call `WeakSet` because in type argument `T`: [incompatible-call]
                                    ^ [1]
 
 References:
-   <BUILTINS>/core.js:1840:28
-   1840| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
+   <BUILTINS>/core.js:1958:28
+   1958| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
                                     ^^^^^^^^^^^^ [2]
-   <BUILTINS>/core.js:1840:43
-   1840| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
+   <BUILTINS>/core.js:1958:43
+   1958| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
                                                    ^^^^^^^^^^^^^^^^^^^^^ [3]
 
 
@@ -306,11 +306,11 @@ Cannot call `WeakSet` because in type argument `T`: [incompatible-call]
                                       ^ [1]
 
 References:
-   <BUILTINS>/core.js:1840:28
-   1840| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
+   <BUILTINS>/core.js:1958:28
+   1958| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
                                     ^^^^^^^^^^^^ [2]
-   <BUILTINS>/core.js:1840:43
-   1840| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
+   <BUILTINS>/core.js:1958:43
+   1958| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
                                                    ^^^^^^^^^^^^^^^^^^^^^ [3]
 
 
@@ -329,11 +329,11 @@ References:
    weakset.js:27:31
      27| function* numbers(): Iterable<number> {
                                        ^^^^^^ [1]
-   <BUILTINS>/core.js:1840:28
-   1840| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
+   <BUILTINS>/core.js:1958:28
+   1958| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
                                     ^^^^^^^^^^^^ [2]
-   <BUILTINS>/core.js:1840:43
-   1840| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
+   <BUILTINS>/core.js:1958:43
+   1958| type WeaklyReferenceable = interface {} | $ReadOnlyArray<mixed>;
                                                    ^^^^^^^^^^^^^^^^^^^^^ [3]
 
 
diff --git a/tests/enums/enums.exp b/tests/enums/enums.exp
index cee5814381a..d4804f94d77 100644
--- a/tests/enums/enums.exp
+++ b/tests/enums/enums.exp
@@ -48,20 +48,20 @@ Cannot instantiate `EnumValue` because in type argument `TRepresentationType`: [
                           ^^^^^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:2858:32
-   2858| type EnumRepresentationTypes = string | number | symbol | boolean | bigint;
+   <BUILTINS>/core.js:2976:32
+   2976| type EnumRepresentationTypes = string | number | symbol | boolean | bigint;
                                         ^^^^^^ [2]
-   <BUILTINS>/core.js:2858:41
-   2858| type EnumRepresentationTypes = string | number | symbol | boolean | bigint;
+   <BUILTINS>/core.js:2976:41
+   2976| type EnumRepresentationTypes = string | number | symbol | boolean | bigint;
                                                  ^^^^^^ [3]
-   <BUILTINS>/core.js:2858:50
-   2858| type EnumRepresentationTypes = string | number | symbol | boolean | bigint;
+   <BUILTINS>/core.js:2976:50
+   2976| type EnumRepresentationTypes = string | number | symbol | boolean | bigint;
                                                           ^^^^^^ [4]
-   <BUILTINS>/core.js:2858:59
-   2858| type EnumRepresentationTypes = string | number | symbol | boolean | bigint;
+   <BUILTINS>/core.js:2976:59
+   2976| type EnumRepresentationTypes = string | number | symbol | boolean | bigint;
                                                                    ^^^^^^^ [5]
-   <BUILTINS>/core.js:2858:69
-   2858| type EnumRepresentationTypes = string | number | symbol | boolean | bigint;
+   <BUILTINS>/core.js:2976:69
+   2976| type EnumRepresentationTypes = string | number | symbol | boolean | bigint;
                                                                              ^^^^^^ [6]
 
 
@@ -143,8 +143,8 @@ Cannot cast `x` to `EnumValue` because number [1] is incompatible with boolean [
            ^
 
 References:
-   <BUILTINS>/core.js:2872:16
-   2872| > = $EnumValue<TRepresentationType>;
+   <BUILTINS>/core.js:2990:16
+   2990| > = $EnumValue<TRepresentationType>;
                         ^^^^^^^^^^^^^^^^^^^ [1]
    abstract-enum-value.js:35:18
      35|   x as EnumValue<boolean>; // ERROR
@@ -161,8 +161,8 @@ Cannot cast `x` to `EnumValue` because string [1] is incompatible with boolean [
            ^
 
 References:
-   <BUILTINS>/core.js:2872:16
-   2872| > = $EnumValue<TRepresentationType>;
+   <BUILTINS>/core.js:2990:16
+   2990| > = $EnumValue<TRepresentationType>;
                         ^^^^^^^^^^^^^^^^^^^ [1]
    abstract-enum-value.js:35:18
      35|   x as EnumValue<boolean>; // ERROR
@@ -232,8 +232,8 @@ Cannot instantiate `Enum` because boolean [1] is incompatible with `EnumValue` [
                        ^^^^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:2889:24
-   2889| type Enum<+TEnumValue: EnumValue<> = EnumValue<>> = $Enum<TEnumValue>;
+   <BUILTINS>/core.js:3007:24
+   3007| type Enum<+TEnumValue: EnumValue<> = EnumValue<>> = $Enum<TEnumValue>;
                                 ^^^^^^^^^^^ [2]
 
 
@@ -270,11 +270,11 @@ References:
    abstract-enum.js:19:23
      19|   x as Enum<EnumValue<boolean>>; // ERROR
                                ^^^^^^^ [2]
-   <BUILTINS>/core.js:2871:4
-   2871|   +TRepresentationType: EnumRepresentationTypes = EnumRepresentationTypes
+   <BUILTINS>/core.js:2989:4
+   2989|   +TRepresentationType: EnumRepresentationTypes = EnumRepresentationTypes
             ^^^^^^^^^^^^^^^^^^^ [3]
-   <BUILTINS>/core.js:2889:12
-   2889| type Enum<+TEnumValue: EnumValue<> = EnumValue<>> = $Enum<TEnumValue>;
+   <BUILTINS>/core.js:3007:12
+   3007| type Enum<+TEnumValue: EnumValue<> = EnumValue<>> = $Enum<TEnumValue>;
                     ^^^^^^^^^^ [4]
 
 
@@ -294,8 +294,8 @@ References:
    abstract-enum.js:20:13
      20|   x as Enum<E>; // ERROR
                      ^ [2]
-   <BUILTINS>/core.js:2889:12
-   2889| type Enum<+TEnumValue: EnumValue<> = EnumValue<>> = $Enum<TEnumValue>;
+   <BUILTINS>/core.js:3007:12
+   3007| type Enum<+TEnumValue: EnumValue<> = EnumValue<>> = $Enum<TEnumValue>;
                     ^^^^^^^^^^ [3]
 
 
@@ -338,8 +338,8 @@ Cannot call `f` because boolean [1] is incompatible with `EnumValue` [2] in type
            ^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:2889:24
-   2889| type Enum<+TEnumValue: EnumValue<> = EnumValue<>> = $Enum<TEnumValue>;
+   <BUILTINS>/core.js:3007:24
+   3007| type Enum<+TEnumValue: EnumValue<> = EnumValue<>> = $Enum<TEnumValue>;
                                 ^^^^^^^^^^^ [2]
 
 
@@ -1629,9 +1629,9 @@ implicitly-returned undefined in type argument `Return` [2]. [incompatible-retur
                                             ^^^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:1756:29
-   1756| interface Generator<+Yield,+Return,-Next> {
-                                     ^^^^^^ [2]
+   <BUILTINS>/core.js:1879:24
+   1879| type Generator<+Yield,+Return,-Next> = $Generator<Yield,Return,Next>;
+                                ^^^^^^ [2]
 
 
 Error ----------------------------------------------------------------------- exhaustive-check-implicit-return.js:202:11
@@ -1659,8 +1659,8 @@ undefined in type argument `R` [2]. [incompatible-return]
                                          ^^^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:1984:24
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:24
+   2102| declare class Promise<+R = mixed> {
                                 ^ [2]
 
 
@@ -1794,8 +1794,8 @@ References:
    exhaustive-check.js:3:6
       3| enum E {
               ^ [2]
-   <BUILTINS>/core.js:2659:3
-   2659|   isValid(this: TEnum, input: ?TRepresentationType | TEnumValue): boolean,
+   <BUILTINS>/core.js:2777:3
+   2777|   isValid(this: TEnum, input: ?TRepresentationType | TEnumValue): boolean,
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ [3]
 
 
@@ -2513,15 +2513,15 @@ Cannot get `E.nonExistent` because property `nonExistent` is missing in `$EnumPr
            ^^^^^^^^^^^
 
 References:
-   <BUILTINS>/core.js:2656:59
+   <BUILTINS>/core.js:2774:59
                                                                    v-
-   2656| type $EnumProto<TEnum, TEnumValue, TRepresentationType> = {|
-   2657|   cast(this: TEnum, input: ?TRepresentationType): void | TEnumValue,
-   2658|   getName(this: TEnum, input: TEnumValue): string,
-   2659|   isValid(this: TEnum, input: ?TRepresentationType | TEnumValue): boolean,
-   2660|   members(this: TEnum): Iterator<TEnumValue>,
-   2661|   __proto__: null,
-   2662| |}
+   2774| type $EnumProto<TEnum, TEnumValue, TRepresentationType> = {|
+   2775|   cast(this: TEnum, input: ?TRepresentationType): void | TEnumValue,
+   2776|   getName(this: TEnum, input: TEnumValue): string,
+   2777|   isValid(this: TEnum, input: ?TRepresentationType | TEnumValue): boolean,
+   2778|   members(this: TEnum): Iterator<TEnumValue>,
+   2779|   __proto__: null,
+   2780| |}
          -^ [1]
 
 
@@ -2534,15 +2534,15 @@ Cannot call `E.nonExistent` because property `nonExistent` is missing in `$EnumP
            ^^^^^^^^^^^
 
 References:
-   <BUILTINS>/core.js:2656:59
+   <BUILTINS>/core.js:2774:59
                                                                    v-
-   2656| type $EnumProto<TEnum, TEnumValue, TRepresentationType> = {|
-   2657|   cast(this: TEnum, input: ?TRepresentationType): void | TEnumValue,
-   2658|   getName(this: TEnum, input: TEnumValue): string,
-   2659|   isValid(this: TEnum, input: ?TRepresentationType | TEnumValue): boolean,
-   2660|   members(this: TEnum): Iterator<TEnumValue>,
-   2661|   __proto__: null,
-   2662| |}
+   2774| type $EnumProto<TEnum, TEnumValue, TRepresentationType> = {|
+   2775|   cast(this: TEnum, input: ?TRepresentationType): void | TEnumValue,
+   2776|   getName(this: TEnum, input: TEnumValue): string,
+   2777|   isValid(this: TEnum, input: ?TRepresentationType | TEnumValue): boolean,
+   2778|   members(this: TEnum): Iterator<TEnumValue>,
+   2779|   __proto__: null,
+   2780| |}
          -^ [1]
 
 
@@ -2569,15 +2569,15 @@ Cannot call `E.A` because property `A` is missing in `$EnumProto` [1]. [prop-mis
            ^
 
 References:
-   <BUILTINS>/core.js:2656:59
+   <BUILTINS>/core.js:2774:59
                                                                    v-
-   2656| type $EnumProto<TEnum, TEnumValue, TRepresentationType> = {|
-   2657|   cast(this: TEnum, input: ?TRepresentationType): void | TEnumValue,
-   2658|   getName(this: TEnum, input: TEnumValue): string,
-   2659|   isValid(this: TEnum, input: ?TRepresentationType | TEnumValue): boolean,
-   2660|   members(this: TEnum): Iterator<TEnumValue>,
-   2661|   __proto__: null,
-   2662| |}
+   2774| type $EnumProto<TEnum, TEnumValue, TRepresentationType> = {|
+   2775|   cast(this: TEnum, input: ?TRepresentationType): void | TEnumValue,
+   2776|   getName(this: TEnum, input: TEnumValue): string,
+   2777|   isValid(this: TEnum, input: ?TRepresentationType | TEnumValue): boolean,
+   2778|   members(this: TEnum): Iterator<TEnumValue>,
+   2779|   __proto__: null,
+   2780| |}
          -^ [1]
 
 
@@ -2590,15 +2590,15 @@ Cannot call `E.toString` because property `toString` is missing in `$EnumProto`
            ^^^^^^^^
 
 References:
-   <BUILTINS>/core.js:2656:59
+   <BUILTINS>/core.js:2774:59
                                                                    v-
-   2656| type $EnumProto<TEnum, TEnumValue, TRepresentationType> = {|
-   2657|   cast(this: TEnum, input: ?TRepresentationType): void | TEnumValue,
-   2658|   getName(this: TEnum, input: TEnumValue): string,
-   2659|   isValid(this: TEnum, input: ?TRepresentationType | TEnumValue): boolean,
-   2660|   members(this: TEnum): Iterator<TEnumValue>,
-   2661|   __proto__: null,
-   2662| |}
+   2774| type $EnumProto<TEnum, TEnumValue, TRepresentationType> = {|
+   2775|   cast(this: TEnum, input: ?TRepresentationType): void | TEnumValue,
+   2776|   getName(this: TEnum, input: TEnumValue): string,
+   2777|   isValid(this: TEnum, input: ?TRepresentationType | TEnumValue): boolean,
+   2778|   members(this: TEnum): Iterator<TEnumValue>,
+   2779|   __proto__: null,
+   2780| |}
          -^ [1]
 
 
@@ -2611,8 +2611,8 @@ Cannot cast `E.getName(...)` to boolean because string [1] is incompatible with
          ^^^^^^^^^^^^^^
 
 References:
-   <BUILTINS>/core.js:2658:44
-   2658|   getName(this: TEnum, input: TEnumValue): string,
+   <BUILTINS>/core.js:2776:44
+   2776|   getName(this: TEnum, input: TEnumValue): string,
                                                     ^^^^^^ [1]
    methods.js:44:19
      44| E.getName(E.B) as boolean; // Error - wrong type
diff --git a/tests/fetch/fetch.exp b/tests/fetch/fetch.exp
index 20b63ac2d32..008ac8d9288 100644
--- a/tests/fetch/fetch.exp
+++ b/tests/fetch/fetch.exp
@@ -14,8 +14,8 @@ References:
    fetch.js:12:18
      12| const b: Promise<string> = fetch(myRequest); // incorrect
                           ^^^^^^ [2]
-   <BUILTINS>/core.js:1984:24
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:24
+   2102| declare class Promise<+R = mixed> {
                                 ^ [3]
 
 
@@ -35,8 +35,8 @@ References:
    <BUILTINS>/bom.js:1733:76
    1733| declare function fetch(input: RequestInfo, init?: RequestOptions): Promise<Response>;
                                                                                     ^^^^^^^^ [2]
-   <BUILTINS>/core.js:1984:24
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:24
+   2102| declare class Promise<+R = mixed> {
                                 ^ [3]
 
 
@@ -564,8 +564,8 @@ References:
    request.js:24:15
      24| h.text().then((t: Buffer) => t); // incorrect
                        ^^^^^^^^^^^^^^^^ [3]
-   <BUILTINS>/core.js:1997:18
-   1997|       onFulfill: null | void,
+   <BUILTINS>/core.js:2115:18
+   2115|       onFulfill: null | void,
                           ^^^^^^^^^^^ [4]
 
 
@@ -589,8 +589,8 @@ References:
    request.js:26:22
      26| h.arrayBuffer().then((ab: Buffer) => ab); // incorrect
                               ^^^^^^^^^^^^^^^^^^ [3]
-   <BUILTINS>/core.js:1997:18
-   1997|       onFulfill: null | void,
+   <BUILTINS>/core.js:2115:18
+   2115|       onFulfill: null | void,
                           ^^^^^^^^^^^ [4]
 
 
@@ -721,17 +721,17 @@ References:
    <BUILTINS>/bom.js:1637:62
    1637| type BodyInit = string | URLSearchParams | FormData | Blob | ArrayBuffer | $ArrayBufferView | ReadableStream;
                                                                       ^^^^^^^^^^^ [5]
-   <BUILTINS>/core.js:2102:20
-   2102| type $TypedArray = $TypedArrayNumber | BigInt64Array | BigUint64Array;
+   <BUILTINS>/core.js:2220:20
+   2220| type $TypedArray = $TypedArrayNumber | BigInt64Array | BigUint64Array;
                             ^^^^^^^^^^^^^^^^^ [6]
-   <BUILTINS>/core.js:2102:40
-   2102| type $TypedArray = $TypedArrayNumber | BigInt64Array | BigUint64Array;
+   <BUILTINS>/core.js:2220:40
+   2220| type $TypedArray = $TypedArrayNumber | BigInt64Array | BigUint64Array;
                                                 ^^^^^^^^^^^^^ [7]
-   <BUILTINS>/core.js:2102:56
-   2102| type $TypedArray = $TypedArrayNumber | BigInt64Array | BigUint64Array;
+   <BUILTINS>/core.js:2220:56
+   2220| type $TypedArray = $TypedArrayNumber | BigInt64Array | BigUint64Array;
                                                                 ^^^^^^^^^^^^^^ [8]
-   <BUILTINS>/core.js:2098:39
-   2098| type $ArrayBufferView = $TypedArray | DataView;
+   <BUILTINS>/core.js:2216:39
+   2216| type $ArrayBufferView = $TypedArray | DataView;
                                                ^^^^^^^^ [9]
    <BUILTINS>/bom.js:1637:95
    1637| type BodyInit = string | URLSearchParams | FormData | Blob | ArrayBuffer | $ArrayBufferView | ReadableStream;
@@ -758,8 +758,8 @@ References:
    response.js:42:15
      42| h.text().then((t: Buffer) => t); // incorrect
                        ^^^^^^^^^^^^^^^^ [3]
-   <BUILTINS>/core.js:1997:18
-   1997|       onFulfill: null | void,
+   <BUILTINS>/core.js:2115:18
+   2115|       onFulfill: null | void,
                           ^^^^^^^^^^^ [4]
 
 
@@ -783,8 +783,8 @@ References:
    response.js:44:22
      44| h.arrayBuffer().then((ab: Buffer) => ab); // incorrect
                               ^^^^^^^^^^^^^^^^^^ [3]
-   <BUILTINS>/core.js:1997:18
-   1997|       onFulfill: null | void,
+   <BUILTINS>/core.js:2115:18
+   2115|       onFulfill: null | void,
                           ^^^^^^^^^^^ [4]
 
 
diff --git a/tests/forof/forof.exp b/tests/forof/forof.exp
index c2b53f1f74a..2e289ff0df0 100644
--- a/tests/forof/forof.exp
+++ b/tests/forof/forof.exp
@@ -45,8 +45,8 @@ References:
    forof.js:23:26
      23| function testString(str: string): void {
                                   ^^^^^^ [1]
-   <BUILTINS>/core.js:1751:11
-   1751| interface $Iterable<+Yield,+Return,-Next> {
+   <BUILTINS>/core.js:1865:11
+   1865| interface $Iterable<+Yield,+Return,-Next> {
                    ^^^^^^^^^ [2]
 
 
@@ -59,8 +59,8 @@ Cannot cast `elem` to number because tuple type [1] is incompatible with number
              ^^^^
 
 References:
-   <BUILTINS>/core.js:1814:28
-   1814|     @@iterator(): Iterator<[K, V]>;
+   <BUILTINS>/core.js:1932:28
+   1932|     @@iterator(): Iterator<[K, V]>;
                                     ^^^^^^ [1]
    forof.js:32:13
      32|     elem as number; // Error - tuple ~> number
diff --git a/tests/function/function.exp b/tests/function/function.exp
index cb47ac5f6ca..e6b5adb4392 100644
--- a/tests/function/function.exp
+++ b/tests/function/function.exp
@@ -43,8 +43,8 @@ it into an object and attempt to use it as a subtype of an interface. [incompati
                         ^^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:1751:11
-   1751| interface $Iterable<+Yield,+Return,-Next> {
+   <BUILTINS>/core.js:1865:11
+   1865| interface $Iterable<+Yield,+Return,-Next> {
                    ^^^^^^^^^ [2]
 
 
@@ -103,7 +103,7 @@ References:
 
 Error ---------------------------------------------------------------------------------------- apply-array-like.js:21:16
 
-Cannot call `test.apply` because property `length` is missing in `Generator` [1] but exists in `$ArrayLike` [2] in type
+Cannot call `test.apply` because property `length` is missing in `$Generator` [1] but exists in `$ArrayLike` [2] in type
 argument `A`. [prop-missing]
 
    apply-array-like.js:21:16
@@ -111,9 +111,9 @@ argument `A`. [prop-missing]
                         ^^^^^
 
 References:
-   <BUILTINS>/core.js:1756:11
-   1756| interface Generator<+Yield,+Return,-Next> {
-                   ^^^^^^^^^ [1]
+   <BUILTINS>/core.js:1879:40
+   1879| type Generator<+Yield,+Return,-Next> = $Generator<Yield,Return,Next>;
+                                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ [1]
    <BUILTINS>/core.js:362:29
     362|     proto apply: (<T, R, A: $ArrayLike<mixed> = []>(this: (this: T, ...args: A) => R, thisArg: T, args?: A) => R);
                                      ^^^^^^^^^^^^^^^^^ [2]
@@ -121,7 +121,7 @@ References:
 
 Error ---------------------------------------------------------------------------------------- apply-array-like.js:21:16
 
-Cannot call `test.apply` because property `length` is missing in `Generator` [1] but exists in `$ArrayLike` [2] in type
+Cannot call `test.apply` because property `length` is missing in `$Generator` [1] but exists in `$ArrayLike` [2] in type
 argument `A`. [prop-missing]
 
    apply-array-like.js:21:16
@@ -242,8 +242,8 @@ it into an object and attempt to use it as a subtype of an interface. [incompati
                         ^^^^^^^^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:1751:11
-   1751| interface $Iterable<+Yield,+Return,-Next> {
+   <BUILTINS>/core.js:1865:11
+   1865| interface $Iterable<+Yield,+Return,-Next> {
                    ^^^^^^^^^ [2]
 
 
diff --git a/tests/generators/generators.exp b/tests/generators/generators.exp
index 034f56dec60..fce30eb770a 100644
--- a/tests/generators/generators.exp
+++ b/tests/generators/generators.exp
@@ -42,9 +42,9 @@ References:
    class.js:23:39
      23|   *stmt_return_err(): Generator<void, number, void> {
                                                ^^^^^^ [2]
-   <BUILTINS>/core.js:1756:29
-   1756| interface Generator<+Yield,+Return,-Next> {
-                                     ^^^^^^ [3]
+   <BUILTINS>/core.js:1879:24
+   1879| type Generator<+Yield,+Return,-Next> = $Generator<Yield,Return,Next>;
+                                ^^^^^^ [3]
 
 
 Error --------------------------------------------------------------------------------------------------- class.js:28:22
@@ -125,15 +125,15 @@ of property `@@iterator`. [incompatible-type-arg]
                     ^^
 
 References:
-   <BUILTINS>/core.js:1749:38
-   1749| type Iterator<+T> = $Iterator<T,void,void>;
-                                              ^^^^ [1]
+   <BUILTINS>/core.js:1759:50
+   1759| declare class Iterator<+Yield,+Return=void,-Next=void> implements $IteratorProtocol<Yield,Return,Next>, $Iterable<Yield,Return,Next> {
+                                                          ^^^^ [1]
    class.js:71:71
      71|   *delegate_next_iterable(xs: Array<number>): Generator<number, void, string> {
                                                                                ^^^^^^ [2]
-   <BUILTINS>/core.js:1745:37
-   1745| interface $Iterator<+Yield,+Return,-Next> extends $Iterable<Yield,Return,Next> {
-                                             ^^^^ [3]
+   <BUILTINS>/core.js:1749:50
+   1749| interface $IteratorProtocol<+Yield,+Return=void,-Next=void> {
+                                                          ^^^^ [3]
 
 
 Error --------------------------------------------------------------------------------------------------- class.js:98:41
@@ -374,9 +374,9 @@ References:
    generators.js:22:46
      22| function *stmt_return_err(): Generator<void, number, void> {
                                                       ^^^^^^ [2]
-   <BUILTINS>/core.js:1756:29
-   1756| interface Generator<+Yield,+Return,-Next> {
-                                     ^^^^^^ [3]
+   <BUILTINS>/core.js:1879:24
+   1879| type Generator<+Yield,+Return,-Next> = $Generator<Yield,Return,Next>;
+                                ^^^^^^ [3]
 
 
 Error ---------------------------------------------------------------------------------------------- generators.js:27:20
diff --git a/tests/get_def_enums/get_def_enums.exp b/tests/get_def_enums/get_def_enums.exp
index dc75a57b128..f2a340b0368 100644
--- a/tests/get_def_enums/get_def_enums.exp
+++ b/tests/get_def_enums/get_def_enums.exp
@@ -28,9 +28,9 @@ library.js:4:3,4:5
 
 main.js:27:13
 Flags:
-[LIB] core.js:2657:3,2657:6
+[LIB] core.js:2775:3,2775:6
 
 main.js:30:13
 Flags:
-[LIB] core.js:2659:3,2659:9
+[LIB] core.js:2777:3,2777:9
 
diff --git a/tests/interface/interface.exp b/tests/interface/interface.exp
index d9432d8a28f..81af3cdd52e 100644
--- a/tests/interface/interface.exp
+++ b/tests/interface/interface.exp
@@ -859,8 +859,8 @@ References:
    string.js:1:18
       1| declare const s: string;
                           ^^^^^^ [1]
-   <BUILTINS>/core.js:1751:11
-   1751| interface $Iterable<+Yield,+Return,-Next> {
+   <BUILTINS>/core.js:1865:11
+   1865| interface $Iterable<+Yield,+Return,-Next> {
                    ^^^^^^^^^ [2]
 
 
diff --git a/tests/iterable/iterable.exp b/tests/iterable/iterable.exp
index 278757644b4..fff8067cbd9 100644
--- a/tests/iterable/iterable.exp
+++ b/tests/iterable/iterable.exp
@@ -14,9 +14,9 @@ References:
    array.js:7:20
       7| ['hi'] as Iterable<number>; // Error string ~> number
                             ^^^^^^ [2]
-   <BUILTINS>/core.js:1745:22
-   1745| interface $Iterator<+Yield,+Return,-Next> extends $Iterable<Yield,Return,Next> {
-                              ^^^^^ [3]
+   <BUILTINS>/core.js:1749:30
+   1749| interface $IteratorProtocol<+Yield,+Return=void,-Next=void> {
+                                      ^^^^^ [3]
 
 
 Error ----------------------------------------------------------------------------------------------------- array.js:8:1
@@ -35,9 +35,9 @@ References:
    array.js:8:23
       8| ['hi', 1] as Iterable<string>; // Error number ~> string
                                ^^^^^^ [2]
-   <BUILTINS>/core.js:1745:22
-   1745| interface $Iterator<+Yield,+Return,-Next> extends $Iterable<Yield,Return,Next> {
-                              ^^^^^ [3]
+   <BUILTINS>/core.js:1749:30
+   1749| interface $IteratorProtocol<+Yield,+Return=void,-Next=void> {
+                                      ^^^^^ [3]
 
 
 Error --------------------------------------------------------------------------------------------- caching_bug.js:21:79
@@ -56,9 +56,9 @@ References:
    caching_bug.js:21:62
      21| function miss_the_cache(x: Array<string | number>): Iterable<string> { return x; }
                                                                       ^^^^^^ [2]
-   <BUILTINS>/core.js:1745:22
-   1745| interface $Iterator<+Yield,+Return,-Next> extends $Iterable<Yield,Return,Next> {
-                              ^^^^^ [3]
+   <BUILTINS>/core.js:1749:30
+   1749| interface $IteratorProtocol<+Yield,+Return=void,-Next=void> {
+                                      ^^^^^ [3]
 
 
 Error ----------------------------------------------------------------------------------------- iterator_result.js:23:31
@@ -106,15 +106,15 @@ value of property `@@iterator`. [incompatible-return]
                   ^^^
 
 References:
-   <BUILTINS>/core.js:1814:28
-   1814|     @@iterator(): Iterator<[K, V]>;
+   <BUILTINS>/core.js:1932:28
+   1932|     @@iterator(): Iterator<[K, V]>;
                                     ^^^^^^ [1]
    map.js:13:55
      13| function mapTest4(map: Map<number, string>): Iterable<string> {
                                                                ^^^^^^ [2]
-   <BUILTINS>/core.js:1745:22
-   1745| interface $Iterator<+Yield,+Return,-Next> extends $Iterable<Yield,Return,Next> {
-                              ^^^^^ [3]
+   <BUILTINS>/core.js:1749:30
+   1749| interface $IteratorProtocol<+Yield,+Return=void,-Next=void> {
+                                      ^^^^^ [3]
 
 
 Error ----------------------------------------------------------------------------------------------------- set.js:14:10
@@ -133,9 +133,9 @@ References:
    set.js:13:47
      13| function setTest4(set: Set<string>): Iterable<number> {
                                                        ^^^^^^ [2]
-   <BUILTINS>/core.js:1745:22
-   1745| interface $Iterator<+Yield,+Return,-Next> extends $Iterable<Yield,Return,Next> {
-                              ^^^^^ [3]
+   <BUILTINS>/core.js:1749:30
+   1749| interface $IteratorProtocol<+Yield,+Return=void,-Next=void> {
+                                      ^^^^^ [3]
 
 
 Error ---------------------------------------------------------------------------------------------------- string.js:3:2
@@ -154,9 +154,9 @@ References:
    string.js:3:29
       3| (new String("hi"): Iterable<number>); // Error - string is a Iterable<string>
                                      ^^^^^^ [2]
-   <BUILTINS>/core.js:1745:22
-   1745| interface $Iterator<+Yield,+Return,-Next> extends $Iterable<Yield,Return,Next> {
-                              ^^^^^ [3]
+   <BUILTINS>/core.js:1749:30
+   1749| interface $IteratorProtocol<+Yield,+Return=void,-Next=void> {
+                                      ^^^^^ [3]
 
 
 
diff --git a/tests/iterable/iterator_result.js b/tests/iterable/iterator_result.js
index a87b9cb8a3b..052e348cd95 100644
--- a/tests/iterable/iterator_result.js
+++ b/tests/iterable/iterator_result.js
@@ -1,6 +1,6 @@
 /* @flow */
 
-function makeIterator1(coin_flip: () => boolean ): Iterator<string> {
+function makeIterator1(coin_flip: () => boolean ): $IteratorProtocol<string> {
   return {
     "@@iterator"() { return makeIterator1(coin_flip); },
     next(): IteratorResult<string, void> {
@@ -14,7 +14,7 @@ function makeIterator1(coin_flip: () => boolean ): Iterator<string> {
   }
 }
 
-function makeIterator2(coin_flip: () => boolean ): Iterator<string> {
+function makeIterator2(coin_flip: () => boolean ): $IteratorProtocol<string> {
   return {
     "@@iterator"() { return makeIterator2(coin_flip); },
     next(): IteratorResult<string, void> {
diff --git a/tests/iterator_helpers/.flowconfig b/tests/iterator_helpers/.flowconfig
new file mode 100644
index 00000000000..6a52ce2dabb
--- /dev/null
+++ b/tests/iterator_helpers/.flowconfig
@@ -0,0 +1,3 @@
+[options]
+no_flowlib=false
+all=true
diff --git a/tests/iterator_helpers/drop.js b/tests/iterator_helpers/drop.js
new file mode 100644
index 00000000000..98227df856d
--- /dev/null
+++ b/tests/iterator_helpers/drop.js
@@ -0,0 +1,10 @@
+/* @flow */
+declare const iterator: Iterator<number>;
+
+// Basic usage
+(iterator.drop(2): Iterator<number>); // OK
+
+// Return type is discarded: https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype.drop
+// "ii. If value is done, return undefined."
+declare const iteratorWithReturn: $Iterator<number, number, void>;
+(iteratorWithReturn.drop(2): $Iterator<number, void, void>); // OK
diff --git a/tests/iterator_helpers/every.js b/tests/iterator_helpers/every.js
new file mode 100644
index 00000000000..f81dacb552a
--- /dev/null
+++ b/tests/iterator_helpers/every.js
@@ -0,0 +1,6 @@
+/* @flow */
+declare const iterator: Iterator<number>;
+declare const predicate: (number) => boolean;
+
+// Basic usage
+(iterator.every(predicate): boolean); // OK
diff --git a/tests/iterator_helpers/filter.js b/tests/iterator_helpers/filter.js
new file mode 100644
index 00000000000..12cf7f6153c
--- /dev/null
+++ b/tests/iterator_helpers/filter.js
@@ -0,0 +1,20 @@
+/* @flow */
+declare const iterator: Iterator<number>;
+
+declare function check(x: number): boolean;
+
+// Basic usage
+(iterator.filter(check): Iterator<number>); // OK
+
+// Return type is discarded: https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype.filter
+// "ii. If value is done, return undefined."
+declare const iteratorWithReturn: $Iterator<number, number, void>;
+(iteratorWithReturn.filter(check): $Iterator<number, void, void>); // OK
+
+// Filters nullish values with Boolean
+declare const iteratorWithNullableValues: Iterator<?number>;
+(iteratorWithNullableValues.filter(Boolean): Iterator<number>); // OK
+
+// Propagates type guards
+declare function guarded(x: ?number): implies x is number;
+(iteratorWithNullableValues.filter(guarded): Iterator<number>); // OK
diff --git a/tests/iterator_helpers/find.js b/tests/iterator_helpers/find.js
new file mode 100644
index 00000000000..eb5621c9afc
--- /dev/null
+++ b/tests/iterator_helpers/find.js
@@ -0,0 +1,11 @@
+/* @flow */
+declare const iterator: Iterator<number>;
+declare const predicate: (number) => boolean;
+
+// Basic usage
+(iterator.find(predicate): ?number); // OK
+
+// Propagates type guards
+declare const mixedIterator: Iterator<number | string>;
+declare function guarded(x: number | string): implies x is number;
+(mixedIterator.find(guarded): ?number); // OK
diff --git a/tests/iterator_helpers/flatmap.js b/tests/iterator_helpers/flatmap.js
new file mode 100644
index 00000000000..5d99a23ab92
--- /dev/null
+++ b/tests/iterator_helpers/flatmap.js
@@ -0,0 +1,11 @@
+/* @flow */
+declare const iterator: Iterator<number>;
+declare function mapper(number): $ReadOnlyArray<string>
+
+// Basic usage
+(iterator.flatMap(mapper): Iterator<string>); // OK
+
+// Return type is discarded: https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype.flatmap
+// "ii. If value is done, return undefined."
+declare const iteratorWithReturn: $Iterator<number, number, void>;
+(iteratorWithReturn.flatMap(mapper): $Iterator<string, void, void>); // OK
diff --git a/tests/iterator_helpers/foreach.js b/tests/iterator_helpers/foreach.js
new file mode 100644
index 00000000000..5215da92c5d
--- /dev/null
+++ b/tests/iterator_helpers/foreach.js
@@ -0,0 +1,5 @@
+/* @flow */
+declare const iterator: Iterator<number>;
+
+// Basic usage
+(iterator.forEach((x: number): mixed => {}): void); // OK
diff --git a/tests/iterator_helpers/from.js b/tests/iterator_helpers/from.js
new file mode 100644
index 00000000000..b67b5ebdbd9
--- /dev/null
+++ b/tests/iterator_helpers/from.js
@@ -0,0 +1,7 @@
+/* @flow */
+// From a built-in iterable
+(Iterator.from([1]): Iterator<number>);
+
+// From a custom iterator implementation
+declare const iterator: $IteratorProtocol<number>;
+(Iterator.from(iterator): Iterator<number>);
diff --git a/tests/iterator_helpers/iterator_helpers.exp b/tests/iterator_helpers/iterator_helpers.exp
new file mode 100644
index 00000000000..2829d581f51
--- /dev/null
+++ b/tests/iterator_helpers/iterator_helpers.exp
@@ -0,0 +1 @@
+Found 0 errors
diff --git a/tests/iterator_helpers/map.js b/tests/iterator_helpers/map.js
new file mode 100644
index 00000000000..9215826abe8
--- /dev/null
+++ b/tests/iterator_helpers/map.js
@@ -0,0 +1,10 @@
+/* @flow */
+declare const iterator: Iterator<number>;
+
+// Basic usage
+(iterator.map(String): Iterator<string>); // OK
+
+// Return type is discarded: https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype.map
+// "ii. If value is done, return undefined."
+declare const iteratorWithReturn: $Iterator<number, number, void>;
+(iteratorWithReturn.map(String): $Iterator<string, void, void>); // OK
diff --git a/tests/iterator_helpers/reduce.js b/tests/iterator_helpers/reduce.js
new file mode 100644
index 00000000000..e4758c76c43
--- /dev/null
+++ b/tests/iterator_helpers/reduce.js
@@ -0,0 +1,15 @@
+/* @flow */
+declare const iterator: Iterator<number>;
+declare const numberReducer: (number,  number) => number;
+declare const stringReducer: (string,  number) => string;
+declare const flexibleReducer: (string | number,  number) => string;
+
+// Basic usage
+(iterator.reduce(numberReducer): number); // OK
+(iterator.reduce(flexibleReducer): string | number); // OK
+(iterator.reduce(stringReducer, ''): string); // OK
+
+// Return type is discarded: https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype.reduce
+// "ii. If value is done, return undefined."
+declare const iteratorWithReturn: $Iterator<number, number, void>;
+(iteratorWithReturn.map(numberReducer): $Iterator<number, void, void>); // OK
diff --git a/tests/iterator_helpers/some.js b/tests/iterator_helpers/some.js
new file mode 100644
index 00000000000..e0781bb4a23
--- /dev/null
+++ b/tests/iterator_helpers/some.js
@@ -0,0 +1,6 @@
+/* @flow */
+declare const iterator: Iterator<number>;
+declare const predicate: (number) => boolean;
+
+// Basic usage
+(iterator.some(predicate): boolean); // OK
diff --git a/tests/iterator_helpers/take.js b/tests/iterator_helpers/take.js
new file mode 100644
index 00000000000..1bb416a6fbc
--- /dev/null
+++ b/tests/iterator_helpers/take.js
@@ -0,0 +1,10 @@
+/* @flow */
+declare const iterator: Iterator<number>;
+
+// Basic usage
+(iterator.take(2): Iterator<number>); // OK
+
+// Return type is discarded: https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype.take
+// "ii. If value is done, return undefined."
+declare const iteratorWithReturn: $Iterator<number, number, void>;
+(iteratorWithReturn.take(2): $Iterator<number, void, void>); // OK
diff --git a/tests/iterator_helpers/toarray.js b/tests/iterator_helpers/toarray.js
new file mode 100644
index 00000000000..c1d17448233
--- /dev/null
+++ b/tests/iterator_helpers/toarray.js
@@ -0,0 +1,5 @@
+/* @flow */
+declare const iterator: Iterator<number>;
+
+// Basic usage
+(iterator.toArray(): Array<number>); // OK
diff --git a/tests/local_inference_annotations/local_inference_annotations.exp b/tests/local_inference_annotations/local_inference_annotations.exp
index b67666e0b4d..46a26709721 100644
--- a/tests/local_inference_annotations/local_inference_annotations.exp
+++ b/tests/local_inference_annotations/local_inference_annotations.exp
@@ -63,8 +63,8 @@ Property `@@iterator` is missing in function [1] but exists in `$Iterable` [2].
                 ^^^^^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:1751:11
-   1751| interface $Iterable<+Yield,+Return,-Next> {
+   <BUILTINS>/core.js:1865:11
+   1865| interface $Iterable<+Yield,+Return,-Next> {
                    ^^^^^^^^^ [2]
 
 
@@ -546,8 +546,8 @@ Property `@@iterator` is missing in function [1] but exists in `$Iterable` [2].
                           ^^^^^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:1751:11
-   1751| interface $Iterable<+Yield,+Return,-Next> {
+   <BUILTINS>/core.js:1865:11
+   1865| interface $Iterable<+Yield,+Return,-Next> {
                    ^^^^^^^^^ [2]
 
 
diff --git a/tests/lti_friendly_errors/lti_friendly_errors.exp b/tests/lti_friendly_errors/lti_friendly_errors.exp
index 93d3210c3b8..f2c26d9eac5 100644
--- a/tests/lti_friendly_errors/lti_friendly_errors.exp
+++ b/tests/lti_friendly_errors/lti_friendly_errors.exp
@@ -8,14 +8,14 @@ in type argument `R` [3]. [incompatible-cast]
           ^
 
 References:
-   <BUILTINS>/core.js:1984:28
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:28
+   2102| declare class Promise<+R = mixed> {
                                     ^^^^^ [1]
    reason_of_inferred_targs.js:2:13
       2| (p: Promise<string>); // error
                      ^^^^^^ [2]
-   <BUILTINS>/core.js:1984:24
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:24
+   2102| declare class Promise<+R = mixed> {
                                 ^ [3]
 
 
diff --git a/tests/mapped_type_utilities/mapped_type_utilities.exp b/tests/mapped_type_utilities/mapped_type_utilities.exp
index 179c9dcc1a6..8695a097ad8 100644
--- a/tests/mapped_type_utilities/mapped_type_utilities.exp
+++ b/tests/mapped_type_utilities/mapped_type_utilities.exp
@@ -153,8 +153,8 @@ Cannot cast `pickedO1.bar` to string because undefined [1] is incompatible with
           ^^^^^^^^^^^^
 
 References:
-   <BUILTINS>/core.js:2834:62
-   2834| type Pick<O: interface {}, Keys: $Keys<O>> = {[key in Keys]: O[key]};
+   <BUILTINS>/core.js:2952:62
+   2952| type Pick<O: interface {}, Keys: $Keys<O>> = {[key in Keys]: O[key]};
                                                                       ^^^^^^ [1]
    pick.js:9:16
       9| (pickedO1.bar: string); // ERROR bar is optional
diff --git a/tests/meta_property/meta_property.exp b/tests/meta_property/meta_property.exp
index e86190d5528..75055053134 100644
--- a/tests/meta_property/meta_property.exp
+++ b/tests/meta_property/meta_property.exp
@@ -8,13 +8,13 @@ Cannot cast `import.meta` to number literal `1` because `Import$Meta` [1] is inc
           ^^^^^^^^^^^
 
 References:
-   <BUILTINS>/core.js:2767:20
+   <BUILTINS>/core.js:2885:20
                             v
-   2767| type Import$Meta = {
-   2768|   [string]: mixed,
-   2769|   url?: string,
-   2770|   ...
-   2771| };
+   2885| type Import$Meta = {
+   2886|   [string]: mixed,
+   2887|   url?: string,
+   2888|   ...
+   2889| };
          ^ [1]
    import_meta.js:5:15
       5| (import.meta: 1); // Error
@@ -31,8 +31,8 @@ Cannot cast `import.meta.XXX` to number literal `1` because mixed [1] is incompa
           ^^^^^^^^^^^^^^^
 
 References:
-   <BUILTINS>/core.js:2768:13
-   2768|   [string]: mixed,
+   <BUILTINS>/core.js:2886:13
+   2886|   [string]: mixed,
                      ^^^^^ [1]
    import_meta.js:6:19
       6| (import.meta.XXX: 1); // Error
diff --git a/tests/new_env/new_env.exp b/tests/new_env/new_env.exp
index 972e92b9d9c..f65acf0e92d 100644
--- a/tests/new_env/new_env.exp
+++ b/tests/new_env/new_env.exp
@@ -205,12 +205,12 @@ Cannot call `Promise` because function [1] requires another argument. [incompati
                      ^^^^^^^
 
 References:
-   <BUILTINS>/core.js:1985:5
+   <BUILTINS>/core.js:2103:5
              v----------------------
-   1985|     constructor(callback: (
-   1986|       resolve: (result: Promise<R> | R) => void,
-   1987|       reject: (error: any) => void
-   1988|     ) => mixed): void;
+   2103|     constructor(callback: (
+   2104|       resolve: (result: Promise<R> | R) => void,
+   2105|       reject: (error: any) => void
+   2106|     ) => mixed): void;
              ----------------^ [1]
 
 
diff --git a/tests/new_merge/new_merge.exp b/tests/new_merge/new_merge.exp
index 4cfe4da857c..406804388b4 100644
--- a/tests/new_merge/new_merge.exp
+++ b/tests/new_merge/new_merge.exp
@@ -299,8 +299,8 @@ Cannot cast `f15()` to empty because `Promise` [1] is incompatible with empty [2
          ^^^^^
 
 References:
-   <BUILTINS>/core.js:1984:15
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:15
+   2102| declare class Promise<+R = mixed> {
                        ^^^^^^^ [1]
    main.js:58:10
      58| f15() as empty;
diff --git a/tests/promises/promises.exp b/tests/promises/promises.exp
index 48904043c9d..a41086be2a2 100644
--- a/tests/promises/promises.exp
+++ b/tests/promises/promises.exp
@@ -7,8 +7,8 @@ Cannot cast `a` to number because string [1] is incompatible with number [2]. [i
             ^
 
 References:
-   <BUILTINS>/core.js:2047:58
-   2047|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
+   <BUILTINS>/core.js:2165:58
+   2165|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
                                                                   ^^^^^^^^^^^^^ [1]
    all.js:11:7
      11|   (a: number);  // Error: string ~> number
@@ -24,8 +24,8 @@ Cannot cast `b` to boolean because number [1] is incompatible with boolean [2].
             ^
 
 References:
-   <BUILTINS>/core.js:2047:58
-   2047|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
+   <BUILTINS>/core.js:2165:58
+   2165|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
                                                                   ^^^^^^^^^^^^^ [1]
    all.js:12:7
      12|   (b: boolean); // Error: number ~> boolean
@@ -41,8 +41,8 @@ Cannot cast `c` to string because boolean [1] is incompatible with string [2]. [
             ^
 
 References:
-   <BUILTINS>/core.js:2047:58
-   2047|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
+   <BUILTINS>/core.js:2165:58
+   2165|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
                                                                   ^^^^^^^^^^^^^ [1]
    all.js:13:7
      13|   (c: string);  // Error: boolean ~> string
@@ -58,8 +58,8 @@ Cannot cast `x` to undefined because boolean [1] is incompatible with undefined
               ^
 
 References:
-   <BUILTINS>/core.js:2047:58
-   2047|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
+   <BUILTINS>/core.js:2165:58
+   2165|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
                                                                   ^^^^^^^^^^^^^ [1]
    all.js:17:9
      17|     (x: void);  // Errors: string ~> void, number ~> void, boolean ~> void
@@ -75,8 +75,8 @@ Cannot cast `x` to undefined because number [1] is incompatible with undefined [
               ^
 
 References:
-   <BUILTINS>/core.js:2047:58
-   2047|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
+   <BUILTINS>/core.js:2165:58
+   2165|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
                                                                   ^^^^^^^^^^^^^ [1]
    all.js:17:9
      17|     (x: void);  // Errors: string ~> void, number ~> void, boolean ~> void
@@ -92,8 +92,8 @@ Cannot cast `x` to undefined because string [1] is incompatible with undefined [
               ^
 
 References:
-   <BUILTINS>/core.js:2047:58
-   2047|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
+   <BUILTINS>/core.js:2165:58
+   2165|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
                                                                   ^^^^^^^^^^^^^ [1]
    all.js:17:9
      17|     (x: void);  // Errors: string ~> void, number ~> void, boolean ~> void
@@ -109,12 +109,12 @@ Cannot call `Promise.all` because function [1] requires another argument. [incom
                  ^^^
 
 References:
-   <BUILTINS>/core.js:2046:5
+   <BUILTINS>/core.js:2164:5
              v----------------------------------------------------
-   2046|     static all<T: Iterable<mixed>>(promises: T): Promise<
-   2047|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
-   2048|       T extends Iterable<infer V> ? Array<Awaited<V>> : any
-   2049|     >;
+   2164|     static all<T: Iterable<mixed>>(promises: T): Promise<
+   2165|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: Awaited<T[K]>} :
+   2166|       T extends Iterable<infer V> ? Array<Awaited<V>> : any
+   2167|     >;
              ^ [1]
 
 
@@ -142,12 +142,12 @@ Cannot call `Promise.allSettled` because function [1] requires another argument.
                  ^^^^^^^^^^
 
 References:
-   <BUILTINS>/core.js:2056:5
+   <BUILTINS>/core.js:2174:5
              v-----------------------------------------------------------
-   2056|     static allSettled<T: Iterable<mixed>>(promises: T): Promise<
-   2057|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: $SettledPromiseResult<Awaited<T[K]>>} :
-   2058|       T extends Iterable<infer V> ? Array<$SettledPromiseResult<Awaited<V>>> : any
-   2059|     >;
+   2174|     static allSettled<T: Iterable<mixed>>(promises: T): Promise<
+   2175|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: $SettledPromiseResult<Awaited<T[K]>>} :
+   2176|       T extends Iterable<infer V> ? Array<$SettledPromiseResult<Awaited<V>>> : any
+   2177|     >;
              ^ [1]
 
 
@@ -176,8 +176,8 @@ Cannot cast `first.status` to empty because string literal `rejected` [1] is inc
                 ^^^^^^^^^^^^
 
 References:
-   <BUILTINS>/core.js:2081:12
-   2081|   +status: 'rejected',
+   <BUILTINS>/core.js:2199:12
+   2199|   +status: 'rejected',
                     ^^^^^^^^^^ [1]
    allSettled.js:34:22
      34|       (first.status: empty) // Error: 'rejected' case was not covered
@@ -193,8 +193,8 @@ Cannot call `second.value.foo` because property `foo` is missing in `Bar` [1]. [
                                       ^^^
 
 References:
-   <BUILTINS>/core.js:2057:80
-   2057|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: $SettledPromiseResult<Awaited<T[K]>>} :
+   <BUILTINS>/core.js:2175:80
+   2175|       T extends $ReadOnlyArray<mixed> ? {[K in keyof T]: $SettledPromiseResult<Awaited<T[K]>>} :
                                                                                         ^^^^^^^^^^^^^ [1]
 
 
@@ -207,8 +207,8 @@ Cannot use function type [1] with fewer than 2 type arguments. [missing-type-arg
          ^^^^^^^^^^^
 
 References:
-   <BUILTINS>/core.js:2074:15
-   2074|     static any<T, Elem: Promise<T> | T>(promises: Iterable<Elem>): Promise<T>;
+   <BUILTINS>/core.js:2192:15
+   2192|     static any<T, Elem: Promise<T> | T>(promises: Iterable<Elem>): Promise<T>;
                        ^^^^^^^^^^^^^^^^^^^^^^^^^ [1]
 
 
@@ -221,8 +221,8 @@ Cannot call `Promise.any` because function [1] requires another argument. [incom
                  ^^^
 
 References:
-   <BUILTINS>/core.js:2074:5
-   2074|     static any<T, Elem: Promise<T> | T>(promises: Iterable<Elem>): Promise<T>;
+   <BUILTINS>/core.js:2192:5
+   2192|     static any<T, Elem: Promise<T> | T>(promises: Iterable<Elem>): Promise<T>;
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ [1]
 
 
@@ -235,8 +235,8 @@ Cannot use function type [1] with fewer than 2 type arguments. [missing-type-arg
          ^^^^^^^^^^^
 
 References:
-   <BUILTINS>/core.js:2074:15
-   2074|     static any<T, Elem: Promise<T> | T>(promises: Iterable<Elem>): Promise<T>;
+   <BUILTINS>/core.js:2192:15
+   2192|     static any<T, Elem: Promise<T> | T>(promises: Iterable<Elem>): Promise<T>;
                        ^^^^^^^^^^^^^^^^^^^^^^^^^ [1]
 
 
@@ -251,8 +251,8 @@ an interface. [incompatible-type]
                                    ^ [1]
 
 References:
-   <BUILTINS>/core.js:2074:51
-   2074|     static any<T, Elem: Promise<T> | T>(promises: Iterable<Elem>): Promise<T>;
+   <BUILTINS>/core.js:2192:51
+   2192|     static any<T, Elem: Promise<T> | T>(promises: Iterable<Elem>): Promise<T>;
                                                            ^^^^^^^^^^^^^^ [2]
 
 
@@ -637,8 +637,8 @@ References:
    resolve_void.js:1:29
       1| (Promise.resolve(): Promise<number>); // error
                                      ^^^^^^ [2]
-   <BUILTINS>/core.js:1984:24
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:24
+   2102| declare class Promise<+R = mixed> {
                                 ^ [3]
 
 
@@ -655,8 +655,8 @@ References:
    resolve_void.js:3:38
       3| (Promise.resolve(undefined): Promise<number>); // error
                                               ^^^^^^ [2]
-   <BUILTINS>/core.js:1984:24
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:24
+   2102| declare class Promise<+R = mixed> {
                                 ^ [3]
 
 
diff --git a/tests/react_16_6/react_16_6.exp b/tests/react_16_6/react_16_6.exp
index 6f32228d666..7fc465a7d48 100644
--- a/tests/react_16_6/react_16_6.exp
+++ b/tests/react_16_6/react_16_6.exp
@@ -101,8 +101,8 @@ References:
     671|           ...
     672|         }>,
                  ^ [2]
-   <BUILTINS>/core.js:1984:24
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:24
+   2102| declare class Promise<+R = mixed> {
                                 ^ [3]
 
 
@@ -129,8 +129,8 @@ References:
     671|           ...
     672|         }>,
                  ^ [2]
-   <BUILTINS>/core.js:1984:24
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:24
+   2102| declare class Promise<+R = mixed> {
                                 ^ [3]
 
 
diff --git a/tests/resolved_env/resolved_env.exp b/tests/resolved_env/resolved_env.exp
index 6b628cc5156..85c84bec8c1 100644
--- a/tests/resolved_env/resolved_env.exp
+++ b/tests/resolved_env/resolved_env.exp
@@ -36,8 +36,8 @@ Cannot yield `42` because number [1], a primitive, cannot be used as a subtype o
                                    ^^ [1]
 
 References:
-   <BUILTINS>/core.js:1789:7
-   1789|     : $Iterable<Yield, Return, Next>;
+   <BUILTINS>/core.js:1907:7
+   1907|     : $Iterable<Yield, Return, Next>;
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ [2]
 
 
diff --git a/tests/return/return.exp b/tests/return/return.exp
index 03ced79b166..e982e9832ac 100644
--- a/tests/return/return.exp
+++ b/tests/return/return.exp
@@ -40,8 +40,8 @@ undefined in type argument `R` [2]. [incompatible-return]
                                       ^^^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:1984:24
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:24
+   2102| declare class Promise<+R = mixed> {
                                 ^ [2]
 
 
diff --git a/tests/signature_verification_failure/signature_verification_failure.exp b/tests/signature_verification_failure/signature_verification_failure.exp
index 8357beed161..1e1e19e832b 100644
--- a/tests/signature_verification_failure/signature_verification_failure.exp
+++ b/tests/signature_verification_failure/signature_verification_failure.exp
@@ -176,8 +176,8 @@ References:
    misc.js:5:18
       5| export const c = {['a' + 'b']: 42};
                           ^^^^^^^^^^^^^^^^^ [1]
-   <BUILTINS>/core.js:1751:11
-   1751| interface $Iterable<+Yield,+Return,-Next> {
+   <BUILTINS>/core.js:1865:11
+   1865| interface $Iterable<+Yield,+Return,-Next> {
                    ^^^^^^^^^ [2]
 
 
diff --git a/tests/symbol/symbol.exp b/tests/symbol/symbol.exp
index 2c85dc48326..25694c41853 100644
--- a/tests/symbol/symbol.exp
+++ b/tests/symbol/symbol.exp
@@ -1,6 +1,6 @@
 Error -------------------------------------------------------------------------------------------------- iterator.js:6:2
 
-Cannot cast `foo[Symbol.iterator]` to boolean because `$Iterator` [1] is incompatible with boolean [2].
+Cannot cast `foo[Symbol.iterator]` to boolean because `Iterator` [1] is incompatible with boolean [2].
 [incompatible-cast]
 
    iterator.js:6:2
diff --git a/tests/type_at_pos_react/type_at_pos_react.exp b/tests/type_at_pos_react/type_at_pos_react.exp
index 0189ef58d9e..c67224b42d1 100644
--- a/tests/type_at_pos_react/type_at_pos_react.exp
+++ b/tests/type_at_pos_react/type_at_pos_react.exp
@@ -11,7 +11,7 @@ lazy_ref.js:19:9 = Promise<
 
 'ExactReactElement_DEPRECATED' defined at (lib) react.js:179:13
 'Node' defined at (lib) react.js:367:22
-'Promise' defined at (lib) core.js:1984:14
+'Promise' defined at (lib) core.js:2102:14
 'Props' defined at exports-component.js:5:5
 
 lazy_ref.js:19:9,19:9
diff --git a/tests/type_guards/type_guards.exp b/tests/type_guards/type_guards.exp
index 166db80a9be..9efb7837ea7 100644
--- a/tests/type_guards/type_guards.exp
+++ b/tests/type_guards/type_guards.exp
@@ -1169,16 +1169,16 @@ Cannot declare a type guard on a(n) generator function. [function-predicate]
 
 Error ------------------------------------------------------------------------------------------------- invalid.js:19:32
 
-`Generator` [1] is incompatible with boolean [2]. [incompatible-type]
+`$Generator` [1] is incompatible with boolean [2]. [incompatible-type]
 
    invalid.js:19:32
      19| function *generator(x: mixed): x is number { // error
                                         ^^^^^^^^^^^ [2]
 
 References:
-   <BUILTINS>/core.js:1756:11
-   1756| interface Generator<+Yield,+Return,-Next> {
-                   ^^^^^^^^^ [1]
+   <BUILTINS>/core.js:1879:40
+   1879| type Generator<+Yield,+Return,-Next> = $Generator<Yield,Return,Next>;
+                                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ [1]
 
 
 Error ------------------------------------------------------------------------------------------------- invalid.js:19:32
@@ -1191,23 +1191,23 @@ it into an object and attempt to use it as a subtype of an interface. [incompati
                                         ^^^^^^^^^^^ [1]
 
 References:
-   <BUILTINS>/core.js:1751:11
-   1751| interface $Iterable<+Yield,+Return,-Next> {
+   <BUILTINS>/core.js:1865:11
+   1865| interface $Iterable<+Yield,+Return,-Next> {
                    ^^^^^^^^^ [2]
 
 
 Error ------------------------------------------------------------------------------------------------- invalid.js:20:10
 
-Cannot return `(typeof x) == "number"` because `Generator` [1] is incompatible with boolean [2]. [incompatible-return]
+Cannot return `(typeof x) == "number"` because `$Generator` [1] is incompatible with boolean [2]. [incompatible-return]
 
    invalid.js:20:10
      20|   return typeof x == "number";
                   ^^^^^^^^^^^^^^^^^^^^
 
 References:
-   <BUILTINS>/core.js:1756:11
-   1756| interface Generator<+Yield,+Return,-Next> {
-                   ^^^^^^^^^ [1]
+   <BUILTINS>/core.js:1879:40
+   1879| type Generator<+Yield,+Return,-Next> = $Generator<Yield,Return,Next>;
+                                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ [1]
    invalid.js:19:32
      19| function *generator(x: mixed): x is number { // error
                                         ^^^^^^^^^^^ [2]
@@ -1247,8 +1247,8 @@ Cannot return `(typeof x) == "number"` because `Promise` [1] is incompatible wit
                   ^^^^^^^^^^^^^^^^^^^^
 
 References:
-   <BUILTINS>/core.js:1984:15
-   1984| declare class Promise<+R = mixed> {
+   <BUILTINS>/core.js:2102:15
+   2102| declare class Promise<+R = mixed> {
                        ^^^^^^^ [1]
    invalid.js:28:33
      28| async function async(x: mixed): x is number { // error