Archetypes incorrectly rely on J2CL name mangling

[niloc132]

The next Vertispan tag of J2CL will have a chance in how name mangling works, which breaks our current archetypes and some unit tests. Instead, example code should always use jsinterop before trying to reference Java symbols from JS.

Unfortunately, with the default flags, this is identified as a compile time warning rather than an error, and won't cause an error until runtime.

Consider backporting this and releasing a 0.20.1 if 0.21 takes too long, to avoid the number of broken apps build by archetypes.

[niloc132]

See also #197, which could help users to catch this class of issue (and others).