Make asBroadcastStream take two callbacks.

tools/dom/src/EventStreamProvider.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of html;
 
 /**
  * Adapter for exposing DOM events as Dart streams.
  */
 class _EventStream<T extends Event> extends Stream<T> {
   final EventTarget _target;
   final String _eventType;
   final bool _useCapture;
 
   _EventStream(this._target, this._eventType, this._useCapture);
 
   // DOM events are inherently multi-subscribers.
-  Stream<T> asBroadcastStream() => this;
+  Stream<T> asBroadcastStream({void onListen(StreamSubscription subscription),
+                               void onCancel(StreamSubscription subscription)})
+      => this;
   bool get isBroadcast => true;
 
   StreamSubscription<T> listen(void onData(T event),
       { void onError(error),
         void onDone(),
         bool cancelOnError}) {
 
     return new _EventStreamSubscription<T>(
         this._target, this._eventType, onData, this._useCapture);
   }
@@ skipping 130 matching lines @@
   const _CustomEventStreamProvider(this._eventTypeGetter);
 
   Stream<T> forTarget(EventTarget e, {bool useCapture: false}) {
     return new _EventStream(e, _eventTypeGetter(e), useCapture);
   }
 
   String getEventType(EventTarget target) {
     return _eventTypeGetter(target);
   }
 }