Now works better with futures that have been up-cast.
CoreLibraryReviewExempt: Rewrite of existing pure Dart function.
Change-Id: Iefd05b9cfd8ff0cc3e27fc1122a670030e0901ff
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/283680
Reviewed-by: Nate Bosch <nbosch@google.com>
Commit-Queue: Lasse Nielsen <lrn@google.com>
