Use sync forwarders for `switchLatest` (dart-lang/stream_transform#30)

Towards dart-lang/stream_transform#24

- Don't return a Future from `onCancel` when the values were already
  done.
- Use `sync: true`
- Drop unnecessary `await new Future(() {})`s
- Set onPause, onResume, and onCancel inside onListen.
2 files changed
tree: 44e74b4198f95fc717c4140225126b41b86fc7c3
  1. pkgs/