Fix throttle broadcast bug (dart-lang/stream_transform#23)

- Switch to `fromHandlers` utility.
- Add a test which fails with the old implementation.
- Add an extra `await new Future(() {})` in the test for handling the
  Stream done event. The new StreamTransformer is not sync like this old
  one - this behavior is still correct.
- Remove extra `handleDone` which matches the default behavior.