Make StreamSubscription be the active part of a string.

It is the one keeping the buffer of events, not the stream.

R=ajohnsen@google.com, floitsch@google.com, jmesserly@google.com

Review URL: https://codereview.chromium.org//14753009

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@23204 260f80e4-7a28-3924-810f-c04153c831b5