- Fix how the manage_deps tool checks for existing branches
- Fix how the manage_deps tool checks for existing branches

- Bump async to 80886150a5e6c58006c8ae5a6c2aa7108638e2a9

Changes:
```
> git log --format="%C(auto) %h %s" c642203..8088615
 https://dart.googlesource.com/async.git/+/8088615 update some StreamController apis to Future<void> from Future<dynamic> (#198)
 https://dart.googlesource.com/async.git/+/e660b9c Let `firstOrNull` extension method wait for `cancel` to complete. (#196)

```

Diff: https://dart.googlesource.com/async.git/+/c64220396e0fa2f7b380590abfedbd25635cd7a0~..80886150a5e6c58006c8ae5a6c2aa7108638e2a9/
Change-Id: I29e47e87f57a1a9b3b94da837edc71c305c90365
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/215461
Auto-Submit: Jake Macdonald <28dccc213a1e50b0de348b9cb82c0229ef6b65f0@google.com>
Commit-Queue: Jake Macdonald <28dccc213a1e50b0de348b9cb82c0229ef6b65f0@google.com>
Reviewed-by: Sigurd Meldgaard <3f12f2c0f1efe4fd67753fcd55cd7bc21e5b85cf@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