Add StreamController.multiplex constructor.
Add StreamController.multiplex constructor.

The "multiplex" stream controller will generate a new stream every
time .stream is read on the controller, and it will send events to
any stream that currently has a listener.
The streams handle their own buffering. Events added before the listen
are ignored.

R=floitsch@google.com

Review URL: https://codereview.chromium.org//16003002

git-svn-id: b90127a0f73590c96be848d3e4cb640efa9a7657@23217 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/scheduled_test,dart-archive/scheduled_test