This CL moves language/unsorted/flatten_test.dart to language/async,
changes it from a multi-test to a regular test, removes some run-time
errors that aren't relevant to the purpose of the test, and removes
a function literal that triggers a type inference anomaly (language
repo issue 3148) which is also not the purpose of this test.
Change-Id: Ib0c84f79476dafd9b6d763062eef687d7432d424
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/309280
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Erik Ernst <eernst@google.com>
