Make asBroadcastStream take two callbacks.

sdk/lib/html/dart2js/html_dart2js.dart

 /// The Dart HTML library.
 library dart.dom.html;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:_collection-dev' hide Symbol;
 import 'dart:html_common';
 import 'dart:indexed_db';
 import 'dart:isolate';
 import 'dart:json' as json;
@@ skipping 26048 matching lines @@
  * 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 3517 matching lines @@
       _position = nextPosition;
       return true;
     }
     _current = null;
     _position = _array.length;
     return false;
   }
 
   T get current => _current;
 }