Remove uses of Path class from tests/standalone.

tests/standalone/io/secure_builtin_roots_database_test.dart

 // Copyright (c) 2012, 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:io";
 import "dart:isolate";
 import "dart:async";
 
 void testGoogleUrl() {
   ReceivePort keepAlive = new ReceivePort();
   HttpClient client = new HttpClient();
   client.getUrl(Uri.parse('https://www.google.com'))
       .then((request) => request.close())
       .then((response)=> response.last)
       .then((_) {
         client.close();
         keepAlive.close();
       });
 }
 
 void InitializeSSL() {
   // If the built-in root certificates aren't loaded, the connection
   // should signal an error.  Even when an external database is loaded,
   // they should not be loaded.
-  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: true);
 }
 
 void main() {
   InitializeSSL();
   testGoogleUrl();
 }