[Class modifiers] Remove use of classes as mixins

[mit-mit]

Pre-Dart 3 any class can be used as a mixin. In Dart 3, only mixin class foo (a declaration that defines both a class and a mixin.) and mixin foo are allowed:
dart-lang/language#1643
https://github.com/dart-lang/language/blob/master/accepted/future-releases/class-modifiers/feature-specification.md

There seems to be a bunch of classes in co19 that use classes as mixins, from this test run:
https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8788634602419351857/+/u/test_results/new_test_failures__logs_

Can we get those updated?

cc @lrhn

[sgrekhov]

@mit-mit please provide a link to the Gerrit changelist

[lrhn]

The CL which should (when it successfully lands) enable this is https://dart-review.googlesource.com/c/sdk/+/284304

If CFE/analyzer actually implements it correctly, there might be some tweaks needed, if they don't actually look at the Function declaration.

[mit-mit]

The mentioned changelist is the same as mentioned in #1874 (comment)

[sgrekhov]

If you don't mind I'll fix if when CL is landed

[sgrekhov]

#1907 added // @dart=2.19 to all legacy mixin tests. I believe this fixed this issue. Closing it