Remove uses of Path class from tests/standalone.

tests/standalone/io/regress_7191_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.
 
 // Regression test for http://dartbug.com/7191.
 
 // Starts a sub-process which in turn starts another sub-process and then closes
 // its standard output. If handles are incorrectly inherited on Windows, this
 // will lead to a situation where the stdout of the first sub-process is never
 // closed which will make this test hang.
 
 import 'dart:io';
 import 'dart:isolate';
+import 'package:path/path.dart';
 
 main() {
   var port = new ReceivePort();
   var executable = Platform.executable;
-  var scriptDir = new Path(Platform.script).directoryPath;
-  var script = scriptDir.append('regress_7191_script.dart').toNativePath();
+  var script = join(dirname(Platform.script), 'regress_7191_script.dart');
   Process.start(executable, [script]).then((process) {
     process.stdin.add([0]);
     process.stdout.listen((_) { },
                            onDone: () { process.stdin.add([0]); });
     process.stderr.listen((_) { });
     process.exitCode.then((exitCode) {
       port.close();
       if (exitCode != 0) throw "Bad exit code";
     });
   });
 }