Remove uses of Path class from tests/standalone.

tests/standalone/io/secure_multiple_client_server_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";
 Future<SecureServerSocket> startServer() {
   return SecureServerSocket.bind(HOST_NAME,
                                  0,
                                  CERTIFICATE).then((server) {
@@ skipping 17 matching lines @@
     socket.close();
     return socket.fold(<int>[], (message, data) => message..addAll(data))
         .then((message) {
           Expect.listEquals("Welcome, client $name".codeUnits, message);
           return server;
         });
   });
 }
 
 void 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');
 
   startServer()
       .then((server) => Future.wait(
           ['able', 'baker', 'charlie', 'dozen', 'elapse']
           .map((name) => testClient(server, name))))
       .then((servers) => servers.first.close());
 }