Rewrite Zone implementation to avoid recursively looking up stuff in the parent chain.

sdk/lib/async/broadcast_stream_controller.dart

 // Copyright (c) 2012, 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 dart.async;
 
 class _BroadcastStream<T> extends _ControllerStream<T> {
   _BroadcastStream(_StreamControllerLifecycle controller) : super(controller);
 
   bool get isBroadcast => true;
@@ skipping 27 matching lines @@
 
   bool _expectsEvent(int eventId) =>
       (_eventState & _STATE_EVENT_ID) == eventId;
 
   void _toggleEventId() {
     _eventState ^= _STATE_EVENT_ID;
   }
 
   bool get _isFiring => (_eventState & _STATE_FIRING) != 0;
 
-  bool _setRemoveAfterFiring() {
+  void _setRemoveAfterFiring() {
     assert(_isFiring);
     _eventState |= _STATE_REMOVE_AFTER_FIRING;
   }
 
   bool get _removeAfterFiring =>
       (_eventState & _STATE_REMOVE_AFTER_FIRING) != 0;
 
   // The controller._recordPause doesn't do anything for a broadcast controller,
   // so we don't bother calling it.
   void _onPause() { }
@@ skipping 141 matching lines @@
       subscription._setRemoveAfterFiring();
     } else {
       assert(!identical(subscription._next, subscription));
       _removeListener(subscription);
       // If we are currently firing an event, the empty-check is performed at
       // the end of the listener loop instead of here.
       if (!_isFiring && _isEmpty) {
         _callOnCancel();
       }
     }
+    return null;
   }
 
   void _recordPause(StreamSubscription<T> subscription) {}
   void _recordResume(StreamSubscription<T> subscription) {}
 
   // EventSink interface.
 
   Error _addEventError() {
     if (isClosed) {
       return new StateError("Cannot add new events after calling close");
@@ skipping 267 matching lines @@
     _pauseCount++;
   }
   void resume() { _resume(null); }
   void _resume(_) {
     if (_pauseCount > 0) _pauseCount--;
   }
   Future cancel() { return new _Future.immediate(null); }
   bool get isPaused => _pauseCount > 0;
   Future asFuture([Object value]) => new _Future();
 }