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..bf5540385b05ff63153e66ce5a4f051ddea5536a 100644
--- a/sdk/lib/async/timer.dart
+++ b/sdk/lib/async/timer.dart
@@ -95,6 +95,21 @@ abstract class Timer {
    */
   void cancel();
 
+  /**
+   * The number of durations preceeding the most recent timer event.

[floitsch, 2017/09/21 23:06:45]

preceding

+   *
+   * 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 too much,
+   * so more than one tick should have happened,
+   * all but the last tick 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.
+   */
+  int get tick;
+
   /**
    * Returns whether the timer is still active.
    *