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

tests/standalone/io/timer_cancel_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");
-
-void testSimpleTimer() {
-  Timer cancelTimer;
-  int repeatTimer;
-
-  void timeoutHandlerUnreachable(Timer timer) {
-    Expect.equals(true, false);
-  }
-
-  void timeoutHandler(Timer timer) {
-    cancelTimer.cancel();
-  }
-
-  void timeoutHandlerRepeat(Timer timer) {
-    repeatTimer++;
-    timer.cancel();
-    Expect.equals(true, repeatTimer == 1);
-  }
-
-  cancelTimer = new Timer(1000, timeoutHandlerUnreachable);
-  cancelTimer.cancel();
-  new Timer(1000, timeoutHandler);
-  cancelTimer = new Timer(2000, timeoutHandlerUnreachable);
-  repeatTimer = 0;
-  new Timer.repeating(1500, timeoutHandlerRepeat);
-}
-
-void testCancelTimerWithSameTime() {
-  var t2;
-  var t1 = new Timer(0, (t) => t2.cancel());
-  t2 = new Timer(0, (t) => t1.cancel());
-}
-
-main() {
-  testSimpleTimer();
-  testCancelTimerWithSameTime();
-}