[tests/nnbd] Fix lib/async/zone_error_callback_test for NNBD strong mode
[tests/nnbd] Fix lib/async/zone_error_callback_test for NNBD strong mode

The following tests began passing:
dartk-strong-linux-release-x64:lib/async/zone_error_callback_test was fixed (RuntimeError -> Pass)

Change-Id: I67118d4023cf60737c2434b005007121dd182aff
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/142402
Reviewed-by: Liam Appelbe <78c2029e58e395825ba7228e218c841bb436a9e9@google.com>
Reviewed-by: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@google.com>
Commit-Queue: Alexander Markov <239e774fd29abc1267878f57afe1a8a03e0ca887@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk