Remove uses of Path class from tests/standalone.

tests/standalone/io/secure_server_client_certificate_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: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";
 
 void testClientCertificate() {
   ReceivePort port = new ReceivePort();
   SecureServerSocket.bind(HOST_NAME,
@@ skipping 87 matching lines @@
       clientDone.then((_) {
         Expect.isTrue(clientError);
         server.close();
         port.close();
       });
     });
   });
 }
 
 void main() {
-  Path scriptDir = new Path(Platform.script).directoryPath;
-  Path certificateDatabase = scriptDir.append('pkcert');
-  SecureSocket.initialize(database: certificateDatabase.toNativePath(),
+  String certificateDatabase = join(dirname(Platform.script), 'pkcert');
+  SecureSocket.initialize(database: certificateDatabase,
                           password: 'dartdart',
                           useBuiltinRoots: false);
 
   testClientCertificate();
   testRequiredClientCertificate();
   testNoClientCertificate();
   testNoRequiredClientCertificate();
 }