Make the root zone's schduleMicrotask and create[Periodic]Timer bind the callback.

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 42 matching lines @@
         duration, Zone.current.bindCallback(callback, runGuarded: true));
   }
 
   /**
    * Creates a new repeating timer.
    *
    * The [callback] is invoked repeatedly with [duration] intervals until
    * canceled with the [cancel] function.
    */
   factory Timer.periodic(Duration duration,
-                                  void callback(Timer timer)) {
+                         void callback(Timer timer)) {
     if (Zone.current == Zone.ROOT) {
       // No need to bind the callback. We know that the root's timer will
       // be invoked in the root zone.
       return Zone.current.createPeriodicTimer(duration, callback);
     }
     return Zone.current.createPeriodicTimer(
         duration, Zone.current.bindUnaryCallback(callback, runGuarded: true));
   }
 
   /**
@@ skipping 17 matching lines @@
    * and the timer has not been canceled.
    *
    * A periodic timer is active if it has not been canceled.
    */
   bool get isActive;
 }
 
 external Timer _createTimer(Duration duration, void callback());
 external Timer _createPeriodicTimer(Duration duration,
                                     void callback(Timer timer));