The existing lib/isolate/stacktrace_message_test was failing and also
incorrectly written due to "!" in front of the expected stringification
of the stacktrace.
Issue https://github.com/dart-lang/sdk/issues/46623
TEST=vm/dart{,_2}/isolates/fast_object_copy2_test, lib{,_2}/isolate/stacktrace_message_test
Change-Id: I169d8fd7a7c3cb3b8c57e00287565e3a5c622acf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/216041
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>