Rename XMLHttpRequest to HttpRequest.

lib/dom/src/_XMLHttpRequestUtils.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.
-
-class _XMLHttpRequestUtils {
-
-  // Helper for factory XMLHttpRequest.get
-  static XMLHttpRequest get(String url,
-                            onSuccess(XMLHttpRequest request),
-                            bool withCredentials) {
-    final request = new XMLHttpRequest();
-    request.open('GET', url, true);
-
-    request.withCredentials = withCredentials;
-
-    // Status 0 is for local XHR request.
-    request.on.readyStateChange.add((e) {
-      if (request.readyState == XMLHttpRequest.DONE &&
-          (request.status == 200 || request.status == 0)) {
-        onSuccess(request);
-      }
-    });
-
-    request.send();
-
-    return request;
-  }
-}