Add ticks counter to 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;
 
 /**
  * A count-down timer that can be configured to fire once or repeatedly.
  *
  * The timer counts down from the specified duration to 0.
@@ skipping 78 matching lines @@
   static void run(void callback()) {
     new Timer(Duration.ZERO, callback);
   }
 
   /**
    * Cancels the timer.
    */
   void cancel();
 
   /**
+   * The number of durations preceeding the most recent timer event.
+   *
+   * The value starts at zero and is incremented each time a timer event
+   * occurs, so each callback will see a larger value than the previous one.
+   *
+   * If a periodic timer with a non-zero duration is delayed so much
+   * that more than one event is due at the time when the timer gets to run,

[floitsch, 2017/08/25 09:16:47]

If a periodic timer with a non-zero duration is delayed too much, ticks in the
past are considered "missed", and no callback is invoked for them. The [tick]
count reflects the number of durations that have passed and not the number of
callback invocations that have happened.

+   * those events are combined into one callback
+   * and the tick counter is incremented by the number of combined events.
+   */
+  int get tick;
+
+  /**
    * 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));
 }