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

tests/standalone/io/print_sync_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:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
 void main() {
   asyncStart();
   Process.run(Platform.executable,
-              [new Uri.file(Platform.script)
-                   .resolve('print_sync_script.dart').toString()])
+              [Platform.script.resolve('print_sync_script.dart').toFilePath()])
       .then((out) {
         asyncEnd();
         Expect.equals(1002, out.stdout.split('\n').length);
       });
 }