Add mixins with superclass constraints, to the 'implementers' list.

[srawlins]

"Implementers" is loose. The list of "implementors" of a class (etc.), C, includes classes (etc.) that extend, implement, mix in, or use-as-a-superclass-constraint, C.

Fixes #3406

Also I move a lot of tests to the unit-test-style class_test.dart.

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[srawlins]

CC @dart-lang/analyzer-team