Adding supported checks for touch event.

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

 // Copyright (c) 2013, 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 {
   factory $CLASSNAME(TouchList touches, TouchList targetTouches,
       TouchList changedTouches, String type,
       {Window view, int screenX: 0, int screenY: 0, int clientX: 0,
       int clientY: 0, bool ctrlKey: false, bool altKey: false,
       bool shiftKey: false, bool metaKey: false}) {
     if (view == null) {
       view = window;
     }
     var e = document.$dom_createEvent("TouchEvent");
     e.$dom_initTouchEvent(touches, targetTouches, changedTouches, type, view,
         screenX, screenY, clientX, clientY, ctrlKey, altKey, shiftKey, metaKey);
     return e;
   }
 $!MEMBERS
+
+  /**
+   * Checks if touch events supported on the current platform.
+   *
+   * Note that touch events are only supported if the user is using a touch
+   * device.
+   */
+  static bool get supported {
+$if DART2JS
+    if (JS('bool', '"ontouchstart" in window')) {
+      return Event._isTypeSupported('TouchEvent');
+    }
+    return false;
+$else
+    // TODO: add equivalent 'ontouchstart' check for Dartium.

[Emily Fortuna, 2013/01/29 18:19:21]

TOOD(blois): ...

+    // Basically, this is a fairly common check and it'd be great if it did not
+    // throw exceptions.
+    return Event._isTypeSupported('TouchEvent');
+$endif
+  }
 }