Fix test failures on Windows

tests/standalone/io/directory_uri_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:io';
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 
 void testFromUri() {
   asyncStart();
+  Directory originalWorkingDirectory = Directory.current;
   Directory.systemTemp.createTemp('directory_uri').then((temp) {
     String dirname = temp.path + '/from_uri';
     Uri dirUri = new Uri.file(dirname);
     Directory dir = new Directory.fromUri(dirUri);
     Expect.isTrue(dirUri.isAbsolute);
     Expect.isTrue(dirUri.path.startsWith('/'));
     dir.createSync();
     Expect.isTrue(new Directory.fromUri(dirUri).existsSync());
     Expect.isTrue(
         new Directory.fromUri(Uri.base.resolveUri(dirUri)).existsSync());
     Directory.current = temp.path;
     Expect.isTrue(new Directory.fromUri(Uri.parse('from_uri')).existsSync());
     Expect.isTrue(
         new Directory.fromUri(Uri.base.resolve('from_uri')).existsSync());
+    Directory.current = originalWorkingDirectory;
     dir.deleteSync();
     temp.deleteSync(recursive: true);
     asyncEnd();
   });
 }
 
 void testFromUriUnsupported() {
   Expect.throws(
       () => new Directory.fromUri(
           Uri.parse('http://localhost:8080/index.html')),
       (e) => e is UnsupportedError);
   Expect.throws(
       () => new Directory.fromUri(Uri.parse('ftp://localhost/tmp/xxx')),
       (e) => e is UnsupportedError);
   Expect.throws(() => new Directory.fromUri(Uri.parse('name#fragment')),
                 (e) => e is UnsupportedError);
 }
 
 void main() {
   testFromUri();
   testFromUriUnsupported();
 }