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

sdk/lib/async/timer.dart

Index: sdk/lib/async/timer.dart
diff --git a/sdk/lib/async/timer.dart b/sdk/lib/async/timer.dart
index 8b2b115149f3c02aa904f2a35b9eb3c4c7289faa..46aed99c4975b237808d1081b01a397f95e16431 100644
--- a/sdk/lib/async/timer.dart
+++ b/sdk/lib/async/timer.dart
@@ -93,8 +93,9 @@ abstract class Timer {
    * 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));