Fix test expectations.
Improve test (overly complex).
Mark tests/isolate as immutable to the presubmit script (like other migrated Dart 1 test directories).
(Gardening)
Change-Id: Iec59c59ca992b3dacafbf82176e4e8bd8c5bd3c5
Reviewed-on: https://dart-review.googlesource.com/42945
Reviewed-by: William Hesse <whesse@google.com>
Lasse R.H. Nielsen