[js_interop] No error if a mixin is annotated with `@JS`, `@anonymous`, or `@staticInterop`

[srujzs]

It isn't quite clear what we should do in this case. In some cases, it may silently work, and in other cases it looks like it fails horrifically e.g.

import 'package:js/js.dart';

@JS()
external void eval(String code);

@JS()
mixin M {
}

class S with M {}

void main() {
}

We haven't really designed older interop around mixins. We should warn against this and make sure we don't treat it as a JS interop type in our transformer.

Note that this is true with the dart:js_interop annotations as well.

[sgrekhov]

There also can be a mixin class. Curruntly we have:

@staticInterop
@JS()
mixin class A {}

@staticInterop
@JS()
class C1 = Object with A; // Ok

@staticInterop
@JS()
class C2 with A {}  // Error: JS interop class 'C2' cannot extend Dart class 'Object with A'.

What should be expected in the case above?

[srujzs]

Mixin classes are interesting. I suspect we may want to disallow them with a warning as well and make sure our transformers/error-checking don't treat them as interoperable. There are some weird corner cases like Dart classes mixing in and extending/implementing JS interop mixin classes to be wary of.