Adding supported checks for touch event.

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 5f23f73f4410c3403af038d492cb1d0f74ac6257..44b37d7a662363f6f47096b58c09eb30bc106eb4 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -22363,6 +22363,14 @@ class TouchEvent extends UIEvent native "*TouchEvent" {
   @DocsEditable
   void $dom_initTouchEvent(TouchList touches, TouchList targetTouches, TouchList changedTouches, String type, Window view, int screenX, int screenY, int clientX, int clientY, bool ctrlKey, bool altKey, bool shiftKey, bool metaKey) native;
 
+
+  /// Checks if this type is supported on the current platform.
+  static bool get supported {
+    if (JS('bool', '"ontouchstart" in window')) {
+      return Event._isTypeSupported('TouchEvent');
+    }
+    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