Rename XMLHttpRequest to HttpRequest.

lib/dom/templates/html/dart2js/factoryprovider_XMLHttpRequest.darttemplate

-// 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.
-
-class $FACTORYPROVIDER {
-  factory XMLHttpRequest() native 'return new XMLHttpRequest();';
-
-  factory XMLHttpRequest.get(String url,
-                                     onSuccess(XMLHttpRequest request)) =>
-      _XMLHttpRequestUtils.get(url, onSuccess, false);
-
-  factory XMLHttpRequest.getWithCredentials(String url,
-                                     onSuccess(XMLHttpRequest request)) =>
-      _XMLHttpRequestUtils.get(url, onSuccess, true);
-}