Don't call _ensureRequestAnimationFrame in Dartium.

Review URL: https://codereview.chromium.org/2117103004 .
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 5c43505..b9da2c4 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -39519,7 +39519,6 @@
    */
   @DomName('Window.cancelAnimationFrame')
   void cancelAnimationFrame(int id) {
-    _ensureRequestAnimationFrame();
     var task = AnimationFrameTask._tasks.remove(id);
     if (task == null) {
       // Assume that the animation frame request wasn't intercepted by a zone.
diff --git a/tools/dom/templates/html/impl/impl_Window.darttemplate b/tools/dom/templates/html/impl/impl_Window.darttemplate
index 9563abb..e2cdaee 100644
--- a/tools/dom/templates/html/impl/impl_Window.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Window.darttemplate
@@ -246,7 +246,9 @@
    */
   @DomName('Window.cancelAnimationFrame')
   void cancelAnimationFrame(int id) {
+$if DART2JS
     _ensureRequestAnimationFrame();
+$endif
     var task = AnimationFrameTask._tasks.remove(id);
     if (task == null) {
       // Assume that the animation frame request wasn't intercepted by a zone.