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

tests/isolate/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.
 
+#library('timer_cancel2_test');
+
 #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();
 }