Update to latest package:watcher
Update to latest package:watcher

Fixes a bug in the null safety migration which did not allow a null
return from custom watcher factories.

Change-Id: Ia44978bc2b612451257386ab7839f18ffcb8623c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/178061
Auto-Submit: Nate Bosch <dc212c74d8afde9c690fdd09e74d3e5c5d5a3de0@google.com>
Commit-Queue: Nate Bosch <dc212c74d8afde9c690fdd09e74d3e5c5d5a3de0@google.com>
Commit-Queue: Jake Macdonald <28dccc213a1e50b0de348b9cb82c0229ef6b65f0@google.com>
Reviewed-by: Jake Macdonald <28dccc213a1e50b0de348b9cb82c0229ef6b65f0@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