This introduces an asyncExpectThrowsWhen to async_helper, similar to
Expect.throwsWhen, to allow specifying semantics under behavior
variations in this test.
We also update await_for_test to no longer skip expectations in dart2js.
Change-Id: Ie147f74f384a0e196e40b75c59fe585f011ede49
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/351721
Reviewed-by: Lasse Nielsen <lrn@google.com>
Reviewed-by: Stephen Adams <sra@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>
