Fix AudioContext constructor for Dartc.

client/dom/scripts/template_wrapping_dom_externs.js

 // Copyright (c) 2011, 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.
 
 /**
  * @fileoverview Dart externs for DOM runtime.
  * @externs
  */
 
 if (false) {
 
 var dart_dom_externs = function(){};
 
 // Fields placed on DOM objects and JavaScript constructor functions.
 dart_dom_externs.prototype._dart;
 dart_dom_externs.prototype._dart_class;
 dart_dom_externs.prototype._dart_localStorage;
 
 // Fields placed on Dart objects by native code.
 dart_dom_externs.prototype.$dom;
 
 // Externs missing from JavaScript back-end.
+Window.prototype.AudioContext;
+Window.prototype.webkitAudioContext;
 Window.prototype.webkitRequestAnimationFrame;
 Window.prototype.webkitCancelRequestAnimationFrame;
 Window.prototype.webkitConvertPointFromPageToNode;
 Window.prototype.webkitConvertPointFromNodeToPage;
 
 // Externs for DOM objects.
 var $NAMESPACE = function(){};
 
 $!PROPS
 
 }