Support timers in DOM isolates.

lib/dom/src/Timer.dart

+// Copyright (c) 2012, 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.
+
+// TODO(antonm): support not DOM isolates too.
+class _Timer implements Timer {
+  final canceller;
+
+  _Timer(this.canceller);
+
+  void cancel() { canceller(); }
+}
+
+_getTimerFactoryClosure() =>
+  (int milliSeconds, void callback(Timer timer), bool repeating) {
+    var maker;
+    var canceller;
+    if (repeating) {
+      maker = window.setInterval;
+      canceller = window.clearInterval;
+    } else {
+      maker = window.setTimeout;
+      canceller = window.clearTimeout;
+    }
+    Timer timer;
+    final int id = maker(() { callback(timer); }, milliSeconds);
+    timer = new _Timer(() { canceller(id); });
+    return timer;
+  };