Remove uses of Path class from tests/standalone.

tests/standalone/io/stdin_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 "package:path/path.dart";
 import "dart:io";
 import "dart:json";
 
 void test(String line, List<String> expected) {
-  var script = new Path(Platform.script).directoryPath;
-  script = script.append("stdin_sync_script.dart");
+  var script = join(dirname(Platform.script), "stdin_sync_script.dart");
   Process.start(Platform.executable,
-                [script.toNativePath()]..addAll(
+                [script]..addAll(
                     expected.map(stringify))).then((process) {
     process.stdin.write(line);
     process.stdin.close();
     process.stderr
         .transform(new StringDecoder())
         .transform(new LineTransformer())
         .fold(new StringBuffer(), (b, d) => b..write(d))
         .then((data) {
           if (data.toString() != '') throw "Bad output: '$data'";
         });
@@ skipping 16 matching lines @@
 
   if (Platform.isWindows) {
     // Windows trim one of the \r.
     test("hej\r\r\r\nhej\r\nhej\r", ['hej\r', 'hej', 'hej\r']);
   } else {
     test("hej\r\r\nhej\r\nhej\r", ['hej\r', 'hej', 'hej\r']);
   }
 
   test("hej", ['hej']);
 }