Rename XMLHttpRequest to HttpRequest.

lib/dom/templates/html/dart2js/factoryprovider_HttpRequest.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 HttpRequest() native 'return new XMLHttpRequest();';
 
-  factory XMLHttpRequest.get(String url,
-                                     onSuccess(XMLHttpRequest request)) =>
-      _XMLHttpRequestUtils.get(url, onSuccess, false);
+  factory HttpRequest.get(String url,
+                                     onSuccess(HttpRequest request)) =>
+      _HttpRequestUtils.get(url, onSuccess, false);
 
-  factory XMLHttpRequest.getWithCredentials(String url,
-                                     onSuccess(XMLHttpRequest request)) =>
-      _XMLHttpRequestUtils.get(url, onSuccess, true);
+  factory HttpRequest.getWithCredentials(String url,
+                                     onSuccess(HttpRequest request)) =>
+      _HttpRequestUtils.get(url, onSuccess, true);
 }