Remove uses of Path class from tests/standalone.

tests/standalone/io/socket_upgrade_to_secure_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.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
 import "package:expect/expect.dart";
+import "package:path/path.dart";
 import "dart:async";
 import "dart:io";
 import "dart:isolate";
 
 const HOST_NAME = "localhost";
 const CERTIFICATE = "localhost_cert";
 
 // This test creates a server and a client connects. After connecting
 // and an optional initial handshake the connection is secured by
 // upgrading to a secure connection The client then writes and the
@@ skipping 183 matching lines @@
 
     connectClient(server.port).then(runClient).then((socket) {
       port.close();
     });
   }
 
   ServerSocket.bind(HOST_NAME, 0).then(serverReady);
 }
 
 main() {
-  Path scriptDir = new Path(Platform.script).directoryPath;
-  Path certificateDatabase = scriptDir.append('pkcert');
-  SecureSocket.initialize(database: certificateDatabase.toNativePath(),
+  var certificateDatabase = join(dirname(Platform.script), 'pkcert');
+  SecureSocket.initialize(database: certificateDatabase,
                           password: 'dartdart',
                           useBuiltinRoots: false);
   test(false, false);
   test(true, false);
   test(false, true);
   test(true, true);
   test(false, true, true);
   test(true, true, true);
 }