Add isActive field on Timer.

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;
 
 abstract class Timer {
 
   /**
    * Creates a new timer.
@@ skipping 39 matching lines @@
    * This function is equivalent to `new Timer(Duration.ZERO, callback)`.
    */
   static void run(void callback()) {
     new Timer(Duration.ZERO, callback);
   }
 
   /**
    * Cancels the timer.
    */
   void cancel();
+
+  /**
+   * 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 Timer _createTimer(Duration duration, void callback());
 external Timer _createPeriodicTimer(Duration duration,
                                     void callback(Timer timer));