Change dart:io Platform.script to return a Uri.

tests/standalone/io/secure_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";
@@ skipping 25 matching lines @@
     socket.close();
     return socket.fold(<int>[], (message, data) => message..addAll(data))
         .then((message) {
           Expect.listEquals("Hello server.".codeUnits, message);
           return server;
         });
   });
 }
 
 void main() {
-  String certificateDatabase = join(dirname(Platform.script), 'pkcert');
+  String certificateDatabase = Platform.script.resolve('pkcert').toFilePath();
   SecureSocket.initialize(database: certificateDatabase,
                           password: 'dartdart');
 
   startEchoServer()
       .then(testClient)
       .then((server) {
         server.close();
       });
 }