Add cancelAnimationFrame

tests/html/request_animation_frame_test.dart

Index: tests/html/request_animation_frame_test.dart
diff --git a/tests/html/request_animation_frame_test.dart b/tests/html/request_animation_frame_test.dart
index 62fb5d777f25d78290ec5a6bf2cd99e78aa40dce..15f6919f2130e6d986d46201ae8872e9c3a64fa3 100644
--- a/tests/html/request_animation_frame_test.dart
+++ b/tests/html/request_animation_frame_test.dart
@@ -17,12 +17,38 @@ main() {
 
   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);
     });
 }