Add isActive field on Timer.

sdk/lib/io/timer_impl.dart

 // Copyright (c) 2013, 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.io;
 
 class _Timer extends LinkedListEntry<_Timer> implements Timer {
   // Disables the timer.
   static const int _NO_TIMER = -1;
 
@@ skipping 32 matching lines @@
   _Timer._internal() {}
 
   void _clear() {
     _callback = null;
     _milliSeconds = 0;
     _wakeupTime = 0;
   }
 
   bool get _repeating => _milliSeconds >= 0;
 
+  bool get isActive => _callback != null;
 
   // Cancels a set timer. The timer is removed from the timer list and if
   // the given timer is the earliest timer the native timer is reset.
   void cancel() {
     _clear();
     // Return if already canceled.
     if (list == null) return;
     assert(!_timers.isEmpty);
     _Timer first = _timers.first;
     unlink();
@@ skipping 73 matching lines @@
       // Trigger all of the pending timers. New timers added as part of the
       // callbacks will be enqueued now and notified in the next spin at the
       // earliest.
       _handling_callbacks = true;
       try {
         for (var timer in pending_timers) {
           // One of the timers in the pending_timers list can cancel
           // one of the later timers which will set the callback to
           // null.
           if (timer._callback != null) {
-            timer._callback(timer);
+            var callback = timer._callback;
+            if (!timer._repeating) {
+              //Mark timer as inactive.
+              timer._callback = null;
+            }
+            callback(timer);
             // Re-insert repeating timer if not canceled.
             if (timer._repeating && timer._callback != null) {
               timer._advanceWakeupTime();
               timer._addTimerToList();
             }
           }
         }
       } finally {
         _handling_callbacks = false;
         _notifyEventHandler();
@@ skipping 19 matching lines @@
 _getTimerFactoryClosure() {
   return (int milliSeconds, void callback(Timer timer), bool repeating) {
     if (repeating) {
       return new _Timer.periodic(milliSeconds, callback);
     }
     return new _Timer(milliSeconds, callback);
   };
 }