Adding supported checks to media events

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 6276aa70662e848dcf08f2e26ffed8a334f8b82f..bae41db443dca3963761172b3b30db0834e7b873 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -10199,6 +10199,18 @@ class Event native "*Event" {
   @DomName('Event.stopPropagation')
   void stopPropagation() native;
 
+
+  /**
+   * 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;
+  }
 }
 // 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
@@ -15826,6 +15838,9 @@ class MediaStreamEvents extends Events {
 @DomName('MediaStreamEvent')
 class MediaStreamEvent extends Event native "*MediaStreamEvent" {
 
+  /// Checks if this type is supported on the current platform.
+  static bool get supported => Event._isTypeSupported('MediaStreamEvent');
+
   @DocsEditable
   @DomName('MediaStreamEvent.stream')
   final MediaStream stream;
@@ -15935,6 +15950,9 @@ class MediaStreamTrackEvents extends Events {
 @DomName('MediaStreamTrackEvent')
 class MediaStreamTrackEvent extends Event native "*MediaStreamTrackEvent" {
 
+  /// Checks if this type is supported on the current platform.
+  static bool get supported => Event._isTypeSupported('MediaStreamTrackEvent');
+
   @DocsEditable
   @DomName('MediaStreamTrackEvent.track')
   final MediaStreamTrack track;