Revert "Add ticks counter to Timer."

tests/lib_2/async/periodic_timer2_test.dart

-// Copyright (c) 2011, 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.
-
-import 'dart:async';
-import 'package:expect/expect.dart';
-import 'package:async_helper/async_helper.dart';
-
-const ms = const Duration(milliseconds: 1);
-
-expectGTE(min, actual, msg) {
-  if (actual >= min) return;
-  Expect._fail(msg.replaceAll('{0}', "$min").replaceAll('{1}', "$actual"));
-}
-
-main() {
-  int interval = 20;
-  asyncStart();
-  var sw = new Stopwatch()..start();
-  int nextTick = 1;
-  new Timer.periodic(ms * interval, (t) {
-    expectGTE(nextTick, t.tick, "tick {1} before expect next tick {0}.");
-    nextTick = t.tick + 1; // Always increment tick by at least one.
-    int time = sw.elapsedMilliseconds;
-    int minTime = interval * t.tick;
-    expectGTE(minTime, time, "Actual time {1} before {0} at tick ${t.tick}");
-    if (t.tick > 20) {
-      t.cancel();
-      asyncEnd();
-    }
-  });
-}