Revert "Update to the latest package:async"
Revert "Update to the latest package:async"

This reverts commit 354d9e256e58cb7d9a472be4f182155d1a480ea6.

Reason for revert: Causes internal test failures (http://b/192429661)

Original change's description:
> Update to the latest package:async
>
> Change-Id: I682d0bfb708eb6ac91ab337a952525cfaefde7ab
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/205342
> Auto-Submit: Nate Bosch <nbosch@google.com>
> Commit-Queue: Nate Bosch <nbosch@google.com>
> Commit-Queue: Jake Macdonald <jakemac@google.com>
> Reviewed-by: Jake Macdonald <jakemac@google.com>

TBR=jakemac@google.com,nbosch@google.com

Change-Id: Iefda140a50e772d959830b79c425ab8794c9922d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/205460
Reviewed-by: Michal Terepeta <150b8621a675a5d55b9991692bfca28a89cfbab1@google.com>
Reviewed-by: Ivan Inozemtsev <485d797b24cb233840f738205d28b40bd33bdaad@google.com>
Reviewed-by: Tess Strickland <cce218d042444252b7ac446d60910fca7732559c@google.com>
Commit-Queue: Michal Terepeta <150b8621a675a5d55b9991692bfca28a89cfbab1@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk