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

sdk/lib/async/timer.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;
 
 /**
  * A count-down timer that can be configured to fire once or repeatedly.
  *
  * The timer counts down from the specified duration to 0.
@@ skipping 75 matching lines @@
 
   /**
    * Returns whether the timer is still active.
    *
    * A non-periodic timer is active if the callback has not been executed,
    * and the timer has not been canceled.
    *
    * A periodic timer is active if it has not been canceled.
    */
   bool get isActive;
+
+  external static Timer _createTimer(Duration duration, void callback());
+  external static Timer _createPeriodicTimer(Duration duration,
+                                             void callback(Timer timer));
 }
 
-external Timer _createTimer(Duration duration, void callback());
-external Timer _createPeriodicTimer(Duration duration,
-                                    void callback(Timer timer));