Use window.location.href in path under dart2js.

pkg/path/test/browser_test.dart

 // Copyright (c) 2013, 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.
 
 import 'dart:html';
 
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'package:path/path.dart' as path;
 
 main() {
   useHtmlConfiguration();
 
   group('new Builder()', () {
     test('uses the current working directory if root is omitted', () {
       var builder = new path.Builder();
       expect(builder.root, window.location.href);
     });
 
     test('uses URL if style is omitted', () {
       var builder = new path.Builder();
       expect(builder.style, path.Style.url);
     });
   });
 
   test('current', () {
     expect(path.current, window.location.href);
   });
 }