Implement concat (dart-lang/stream_transform#6)

Closes dart-lang/stream_transform#5

concat implementation and tests. Relatively few tests since most
behavior is through StreamController.addStream and we have high
confidence in that method. Test the thing that is easy to get wrong in
this utility - stream closing.

Add missing README entry for already added debounce.
6 files changed
tree: 6e0392782b973295d19fadfec43f87f6a95e23b8
  1. pkgs/