Remove uses of Path class from tests/standalone.

tests/standalone/io/secure_server_closing_test.dart

Index: tests/standalone/io/secure_server_closing_test.dart
diff --git a/tests/standalone/io/secure_server_closing_test.dart b/tests/standalone/io/secure_server_closing_test.dart
index 67b70fd22d7d6db20acf3017f23edb0e1845341c..da2865015256369f09d0a6a3ec6b7b6a2a164228 100644
--- a/tests/standalone/io/secure_server_closing_test.dart
+++ b/tests/standalone/io/secure_server_closing_test.dart
@@ -8,6 +8,7 @@
 // VMOptions=--short_socket_read --short_socket_write
 
 import "package:expect/expect.dart";
+import "package:path/path.dart";
 import "dart:async";
 import "dart:io";
 import "dart:isolate";
@@ -151,9 +152,8 @@ testCloseServer() {
 
 
 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);