- 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 <[email protected]>
Commit-Queue: Jake Macdonald <[email protected]>
Reviewed-by: Sigurd Meldgaard <[email protected]>

Author: jakemac53

DEPS

@@ -74,7 +74,7 @@ vars = {
 
   # Revisions of /third_party/* dependencies.
   "args_rev": "3b3f55766af13d895d2020ec001a28e8dc147f91",
-  "async_rev": "c64220396e0fa2f7b380590abfedbd25635cd7a0",
+  "async_rev": "80886150a5e6c58006c8ae5a6c2aa7108638e2a9",
   "bazel_worker_rev": "0885637b037979afbf5bcd05fd748b309fd669c0",
   "benchmark_harness_rev": "c546dbd9f639f75cd2f75de8df2eb9f8ea15e8e7",
   "boolean_selector_rev": "665e6921ab246569420376f827bff4585dff0b14",