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

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 "dart:async";
 import "dart:io";
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 import "package:path/path.dart";
@@ skipping 53 matching lines @@
         serverEnd.close();
         server.close();
         completer.complete();
       });
     });
   });
   return completer.future;
 }
 
 void main() {
-  String certificateDatabase = join(dirname(Platform.script), 'pkcert');
+  String certificateDatabase = Platform.script.resolve('pkcert').toFilePath();
   SecureSocket.initialize(database: certificateDatabase,
                           password: 'dartdart',
                           useBuiltinRoots: false);
 
   asyncStart();
   testClientCertificate()
     .then((_) => testRequiredClientCertificate())
     .then((_) => asyncEnd());
 }