fix polymer tests on windows

pkg/polymer/test/paths_test.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.
 
 /** Tests for [PathMapper]. */
 library path_info_test;
 
 import 'package:path/path.dart' as path;
-import 'package:unittest/compact_vm_config.dart';
-import 'package:unittest/unittest.dart';
 import 'package:polymer/src/info.dart';
 import 'package:polymer/src/paths.dart';
 import 'package:polymer/src/utils.dart' as utils;
+import 'package:unittest/compact_vm_config.dart';
+import 'package:unittest/unittest.dart';
 
 main() {
   useCompactVMConfiguration();
   group('outdir == basedir:', () {
+    setUp(() {
+      utils.path = new path.Builder(style: path.Style.posix);
+    });
+
+    tearDown(() {
+      utils.path = new path.Builder();
+    });
+
     group('outputPath', () {
       test('mangle automatic', () {
         var pathMapper = _newPathMapper('a', 'a', false);
         var file = _mockFile('a/b.dart', pathMapper);
         expect(file.dartCodeUrl.resolvedPath, 'a/b.dart');
         expect(pathMapper.outputPath(file.dartCodeUrl.resolvedPath, '.dart'),
             'a/_b.dart.dart');
       });
 
       test('within packages/', () {
@@ skipping 288 matching lines @@
   var file = new FileInfo(new UrlInfo(
         url == null ? filePath : url, filePath, null));
   file.outputFilename = pathMapper.mangle(
       utils.path.basename(filePath), '.dart', false);
   return file;
 }
 
 _checkPath(String filePath, String expected) {
   expect(utils.path.normalize(filePath), expected);
 }