Revert "Revert "Add cancelAnimationFrame""

tests/html/request_animation_frame_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('RequestAnimationFrameTest');
 #import('../../lib/unittest/unittest.dart');
 #import('../../lib/unittest/html_config.dart');
 #import('dart:html');
 
 main() {
   useHtmlConfiguration();
 
   test('oneShot', () {
       var frame = window.requestAnimationFrame(
           expectAsync1((timestamp) { }));
     });
 
   test('twoShot', () {
       var frame = window.requestAnimationFrame(
-          (timestamp1) {
+          expectAsync1((timestamp1) {
             window.requestAnimationFrame(
                 expectAsync1((timestamp2) {
-                  // Not monotonic on Safari and IE.
-                  // Expect.isTrue(timestamp2 > timestamp1, 'timestamps ordered');
-                }));
+                    // Not monotonic on Safari and IE.
+                    // Expect.isTrue(timestamp2 > timestamp1, 'timestamps ordered');
+                  }));
+          }));
+    });
+
+
+  // How do we test that a callback is never called?  We can't wrap the uncalled
+  // callback with 'expectAsync1'.  Will request several frames and try
+  // cancelling the one that is not the last.
+  test('cancel1', () {
+      var frame1 = window.requestAnimationFrame(
+          (timestamp1) {
+            throw new Exception('Should have been cancelled');
           });
+      var frame2 = window.requestAnimationFrame(
+          expectAsync1((timestamp2) { }));
+      window.cancelAnimationFrame(frame1);
+    });
+
+  test('cancel2', () {
+      var frame1 = window.requestAnimationFrame(
+          expectAsync1((timestamp1) { }));
+      var frame2 = window.requestAnimationFrame(
+          (timestamp2) {
+            throw new Exception('Should have been cancelled');
+          });
+      var frame3 = window.requestAnimationFrame(
+          expectAsync1((timestamp3) { }));
+      window.cancelAnimationFrame(frame2);
     });
 }