Update dart:io tests to use asyncStart/asyncEnd

tests/standalone/io/directory_chdir_test.dart

Index: tests/standalone/io/directory_chdir_test.dart
diff --git a/tests/standalone/io/directory_chdir_test.dart b/tests/standalone/io/directory_chdir_test.dart
index 0805e3a58ad4aecd813bce93c6b11efc069b67a5..e1c9a1d14ad24a4a71c3c503a904a10478f4cf8a 100644
--- a/tests/standalone/io/directory_chdir_test.dart
+++ b/tests/standalone/io/directory_chdir_test.dart
@@ -6,12 +6,12 @@
 
 
 import "dart:io";
-import "dart:isolate";
 
+import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 
 testChangeDirectory() {
-  var port = new ReceivePort();
+  asyncStart();
   new Directory("").createTemp().then((temp) {
     var initialCurrent = Directory.current;
     Directory.current = temp;
@@ -28,7 +28,7 @@ testChangeDirectory() {
     Expect.isTrue(new File("222/333").existsSync());
     Directory.current = initialCurrent;
     temp.deleteSync(recursive: true);
-    port.close();
+    asyncEnd();
   });
 }