Provide guidance about package:fake_async vs. FakeAsync from quiver

[jamesderlin]

It's confusing that both package:fake_async and package:quiver both provide mostly-similar-but-not-the-same FakeAsync classes and that both are owned by the Dart team. I tried digging into the history, but it seems rather muddled (it looks like it was added to quiver first, and then a few months later the creator forked it into a separate package without clear explanation why). And while package:fake_async at some point was backwards compatible with quiver's, there's been some divergence.

Should package:fake_async guide people to use the quiver version instead (since that one seems to be maintained, is what Flutter uses, and has more inertia)?

[jamesderlin]

CC @jakemac53 since flutter_test is moving in the opposite direction.

[jamesderlin]

Closing in favor of google/quiver-dart#590.