This includes:
* tests extending non-base class
* tests for applying mixins
* generalization of top-level test to include methods and gettters
* tests for annotating entities indirectly (reexport, hidden
implementation)
* test for using core libraries and core language features.
Change-Id: I2ff928c15794547368d2871f5b982685593ef662
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/396105
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>
Alexander Markov