Improve the handling of code where the async flow forks:
e.g. for `fut.whenComplete(c.complete)` we might proceed
unwinding through awaiters of `c.future` and forget about the
future returned from `whenComplete`. This happens because
we choose to present result of the unwinding as a single
stack and not a tree. In this situation the error will only
propagate into that future and whether or not the error will be
handled depends on that future alone.
As part of this change also start respecting ignored bit on
futures without listeners.
TEST=pkg/vm_service/test/pause_on_unhandled_async_exceptions7_test.dart
CoreLibraryReviewExempt: Comment only changes to core library
Change-Id: I27b689ab07a725e8faa8d91cf40e88ebc8c441a0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/352904
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>
2280b06202