Revert "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 bf5540385b05ff63153e66ce5a4f051ddea5536a..97f68983e31bb2bdcfbfa9afe50cb647f5b18464 100644
--- a/sdk/lib/async/timer.dart
+++ b/sdk/lib/async/timer.dart
@@ -95,21 +95,6 @@ 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 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.
    *