co19 tests - add tests for enhanced enums

[devoncarew]

No description provided.

[devoncarew]

(I haven't looked to see whether we already have these tests)

[eernstg]

@sgrekhov, @iarkh, could you add a comment here when you consider these tests done?

[sgrekhov]

@devoncarew @eernstg, for now tests for Enhanced Enums are mainly completed. Spec was slightly changed and I need to check these changes and update the tests accordingly. I'll let you know when I'll finish with it. Don't thuink that it'll take much time

[sgrekhov]

@devoncarew @eernstg I believe that Enhanced ennums tests are ready. But I'm expecting a change in the specification which should forbid allow user-written index and values. So, not closing this issue yet

[eernstg]

Very good! The spec update is taking place in dart-lang/language#2080.

[eernstg]

Said spec update has just been landed.

[sgrekhov]

@eernstg @devoncarew I believe Enhanced Enums tests are completed.

[eernstg]

We still need to settle whether the following is an error:

abstract class I {
  Never get index;
}

mixin M on Enum implements I {}

The point is that M cannot ever be part of a concrete class, because that class would have to implement Never get index, and it would have to be an enum, and hence it would be an error to override index.

The alternative is that we could declare the error when it arises, in each enum that uses with M or implements M. Those errors will be reported anyway, that should not be news. But it is usually helpful if we can report errors early rather than late, so it would be nice if we could flag M as an error.

[leafpetersen]

I believe no further work here is planned.