We don't want to silently pass tests that think they are async when
the test framework doesn't know it's async.
Change-Id: I6556f8223d8d930ba252496adf233a5cb1108929
Reviewed-on: https://dart-review.googlesource.com/27923
Commit-Queue: Bob Nystrom <rnystrom@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>