Update dart:io tests to use asyncStart/asyncEnd

tests/standalone/io/file_non_ascii_test.dart

Index: tests/standalone/io/file_non_ascii_test.dart
diff --git a/tests/standalone/io/file_non_ascii_test.dart b/tests/standalone/io/file_non_ascii_test.dart
index 3aa0a3a1c4ec63af14343b41fc8e1b3ebd35f8c5..e6e3a75a89f57afa2dcc8eaa66f6139dc7dab429 100644
--- a/tests/standalone/io/file_non_ascii_test.dart
+++ b/tests/standalone/io/file_non_ascii_test.dart
@@ -2,12 +2,13 @@
 // 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 'dart:io';
-import 'dart:isolate';
+
+import "package:async_helper/async_helper.dart";
+import "package:expect/expect.dart";
 
 main() {
-  ReceivePort port = new ReceivePort();
+  asyncStart();
 
   // On MacOS you get the decomposed utf8 form of file and directory
   // names from the system. Therefore, we have to check for both here.
@@ -37,7 +38,7 @@ main() {
                         Expect.isTrue(path.endsWith('${precomposed}.txt') ||
                                       path.endsWith('${decomposed}.txt'));
                         tempDir.delete(recursive: true).then((_) {
-                        port.close();
+                        asyncEnd();
                       });
                     });
                   });