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

tests/isolate/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.
 
+#library('timer_cancel_test');
+
 #import("dart:isolate");
 
 void testSimpleTimer() {
   Timer cancelTimer;
   int repeatTimer;
 
   void timeoutHandlerUnreachable(Timer timer) {
     Expect.equals(true, false);
   }
 
@@ skipping 18 matching lines @@
 void testCancelTimerWithSameTime() {
   var t2;
   var t1 = new Timer(0, (t) => t2.cancel());
   t2 = new Timer(0, (t) => t1.cancel());
 }
 
 main() {
   testSimpleTimer();
   testCancelTimerWithSameTime();
 }