Change dart:io Platform.script to return a Uri.

tests/standalone/io/windows_environment_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.
 
 import 'package:path/path.dart';
 import 'package:expect/expect.dart';
 import "dart:io";
 
 main() {
   if (Platform.operatingSystem != 'windows') return;
   var tempDir = Directory.systemTemp.createTempSync('dart_windows_environment');
   var funkyDir = new Directory(join(tempDir.path, 'å'));
   funkyDir.createSync();
   var funkyFile = new File(join(funkyDir.path, 'funky.bat'));
   funkyFile.writeAsStringSync("""
 @echo off
 set SCRIPTDIR=%~dp0
 %1 %2
       """);
   var dart = Platform.executable;
-  var script = join(dirname(Platform.script),
-                    'windows_environment_script.dart');
+  var script = Platform.script
+                       .resolve('windows_environment_script.dart')
+                       .toFilePath();
   Process.run('cmd',
               ['/c', funkyFile.path, dart, script]).then((p) {
     if (0 != p.exitCode) throw "Exit code not 0";
     tempDir.deleteSync(recursive: true);
   });
 }