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

sdk/lib/async/stream_impl.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;
 
 /** Abstract and private interface for a place to put events. */
 abstract class _EventSink<T> {
   void _add(T data);
   void _addError(Object error, StackTrace stackTrace);
@@ skipping 282 matching lines @@
   void _onPause() {
     assert(_isInputPaused);
   }
 
   void _onResume() {
     assert(!_isInputPaused);
   }
 
   Future _onCancel() {
     assert(_isCanceled);
+    return null;
   }
 
   // Handle pending events.
 
   /**
    * Add a pending event.
    *
    * If the subscription is not paused, this also schedules a firing
    * of pending events later (if necessary).
    */
@@ skipping 753 matching lines @@
       _Future<bool> hasNext = _futureOrPrefetch;
       _clear();
       hasNext._complete(false);
       return;
     }
     _subscription.pause();
     _futureOrPrefetch = null;
     _state = _STATE_EXTRA_DONE;
   }
 }