Remove vestigial constructors in Frog implementation classes.

client/dom/src/frog_FactoryProviders.dart

 // 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.
 
-
-// These factory methods could all live in one factory provider class but dartc
-// has a bug (5399939) preventing that.
-
 class _AudioContextFactoryProvider {
 
   factory AudioContext() native '''
     var constructor = window.AudioContext || window.webkitAudioContext;
     return new constructor();
 ''';
 }
 
 class _DOMParserFactoryProvider {
 
@@ skipping 69 matching lines @@
 
 class _WebSocketFactoryProvider {
 
   factory WebSocket(String url) native '''return new WebSocket(url);''';
 }
 
 class _XMLHttpRequestFactoryProvider {
 
   factory XMLHttpRequest() native '''return new XMLHttpRequest();''';
 }