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();
+    }
+  });
+}