Special case single-subscriptions to broadcast stream controller streams.
Special case single-subscriptions to broadcast stream controller streams.

This reduces the overhead of sending events when there is only a single
listener on the broadcast controller's stream, because it doesn't have to
keep track of which listeners have been notified so far.

R=floitsch@google.com, sgjesse@google.com

Review URL: https://codereview.chromium.org//101873002

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@30847 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk