Adding supported checks to media events

tools/dom/templates/html/impl/impl_Event.darttemplate

 // 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.
 
 // WARNING: Do not edit - generated code.
 
 part of html;
 
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   // In JS, canBubble and cancelable are technically required parameters to
   // init*Event. In practice, though, if they aren't provided they simply
   // default to false (since that's Boolean(undefined)).
   //
   // Contrary to JS, we default canBubble and cancelable to true, since that's
   // what people want most of the time anyway.
   factory $CLASSNAME(String type, [bool canBubble = true, bool cancelable = true]) =>
       _$(CLASSNAME)FactoryProvider.create$CLASSNAME(type, canBubble, cancelable);
 $!MEMBERS
+
+  /**
+   * Checks to see if the event class is supported by the current platform.
+   */
+  static bool _isTypeSupported(String eventType) {
+    // Browsers throw for unsupported event names.
+    try {
+      var e = document.$dom_createEvent(eventType);
+      return e is Event;
+    } catch (_) { }
+    return false;
+  }
 }