Decide how to migrate tests using classes as mixins

[munificent]

Dart 3.0 makes it an error to use a class as a mixin unless the class is marked mixin. This breaks a fairly large number of tests. For now, we're going to mark those tests as @dart=2.19 to opt them out of 3.0 (on the assumption that most of the classes the tests are mixing in are declared in the same library. This should let us enable the class-modifiers experiment.

Eventually, we need to decide what to do with those tests. For each one, it will be some combination of:

1. Turn the class its mixing in into a normal mixin declaration.
2. Add mixin to the class its mixing in.
3. Leave the test at 2.19 as a test of the pre 3.0-behavior, which is still supported by the SDK.

cc @jakemac53