Add ticks counter to Timer.

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 97f68983e31bb2bdcfbfa9afe50cb647f5b18464..f2913b049b07d82a4745f4c560405e4853c988ff 100644
--- a/sdk/lib/async/timer.dart
+++ b/sdk/lib/async/timer.dart
@@ -95,6 +95,19 @@ abstract class 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.
    *