FakeAsync is passing a test after throw UnimplementedError

[omensight]

Hello, this is my test.

test('Join assembly post a join request in server', () {                            
  FakeAsync().run((async) async {                                                   
    final membersDao = MockAssemblyMembersDao();                                    
    final membersApiClient = MockAssemblyMembersApiClient();                        
    final assemblyRequestDao = MockAssemblyJoin RequestsDao();                      
    final userDao = MockUserDao();                                                  
    when(() => membersApiClient.postJoinRequest(any())).thenAnswer(                 
      (invocation) => Future(                                                       
        () => HttpResponse(                                                         
          '',                                                                       
          Response(                                                                 
            requestOptions: RequestOptions(                                         
              path: '',                                                             
            ), // RequestOptions                                                    
            statusCode: 201), // Response                                           
        ), // HttpResponse                                                          
      ), // Future                                                                  
    );                                                                              
    final membersRepository = AssemblyMembersRepository(                            
      assemblyMembersDao: membersDao,                                               
      assemblyMembersApiClient: membersApiClient,                                   
      assemblyJoinRequestsDao: assemblyRequestDao,                                  
      userDao: userDao);                                                            
    var result = await membersRepository.postJoinRequest('uuid');                   
    expect(result, isA>());                                                         
  });                                                                               
);

And this is the not yet implemented method:

Future<Either<RestFailure, Assembly>> postJoinRequest(String uuidCode) async {
  throw UnimplementedError();
}

The test unexpectedly pass instead to show me an error and mark itself red.

[cbracken]

@omensight in order to investigate this, we'll need a reduced repro that can be run without the rest of your codebase. Can you please replace the code above with a minimal repro? Please attach actual code snippets, as opposed to screenshots. I've replaced the screenshots that were attached to the original issue.