Add Timer implementation for dart2js. Also moves tests to appropriate location.

tests/standalone/io/timer_cancel2_test.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.
-
-#import("dart:isolate");
-
-// Test that a timeout handler can cancel itself.
-class TimerCancel2Test {
-  static void testSelfCancel() {
-    var cancelTimer;
-
-    void cancelHandler(Timer timer) {
-      cancelTimer.cancel();
-    }
-
-    cancelTimer = new Timer.repeating(1, cancelHandler);
-  }
-
-  static void testMain() {
-    testSelfCancel();
-  }
-}
-
-main() {
-  TimerCancel2Test.testMain();
-}