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

tests/standalone/io/secure_unauthorized_test.dart

Index: tests/standalone/io/secure_unauthorized_test.dart
diff --git a/tests/standalone/io/secure_unauthorized_test.dart b/tests/standalone/io/secure_unauthorized_test.dart
index 400ae7eb1c49b448b0d675115c74c9971c9b20bf..453b8b1bd4bc57c5dca288096c2355fddd116cee 100644
--- a/tests/standalone/io/secure_unauthorized_test.dart
+++ b/tests/standalone/io/secure_unauthorized_test.dart
@@ -15,7 +15,7 @@ const CERTIFICATE = "localhost_cert";
 
 Future<SecureServerSocket> runServer() {
   SecureSocket.initialize(
-      database: join(dirname(Platform.script), 'pkcert'),
+      database: Platform.script.resolve('pkcert').toFilePath(),
       password: 'dartdart');
 
   return SecureServerSocket.bind(HOST_NAME, 0, CERTIFICATE)
@@ -31,8 +31,9 @@ Future<SecureServerSocket> runServer() {
 }
 
 void main() {
-  var clientScript = join(dirname(Platform.script),
-                          'secure_unauthorized_client.dart');
+  var clientScript = Platform.script
+                             .resolve('secure_unauthorized_client.dart')
+                             .toFilePath();
 
   Future clientProcess(int port) {
     return Process.run(Platform.executable,