Update dart:io tests to use asyncStart/asyncEnd

tests/standalone/io/directory_list_nonexistent_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.
 //
 // Directory listing test that tests listSync on a missing directory.
 //
 // TODO(7157): Merge this test into directory_test.dart testListNonExistent()
 // when it no longer crashes on Windows, when issue 7157 is resolved.
 
+import "dart:io";
+
+import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
-import "dart:io";
-import "dart:isolate";
 
 void testListNonExistent() {
-  var keepAlive = new ReceivePort();
+  asyncStart();
   new Directory("").createTemp().then((d) {
     d.delete().then((ignore) {
       Expect.throws(() => d.listSync(), (e) => e is DirectoryException);
       Expect.throws(() => d.listSync(recursive: true),
                     (e) => e is DirectoryException);
-      keepAlive.close();
+      asyncEnd();
     });
   });
 }
 
 void testListTooLongName() {
-  var keepAlive = new ReceivePort();
+  asyncStart();
   new Directory("").createTemp().then((d) {
     var subDirName = 'subdir';
     var subDir = new Directory("${d.path}/$subDirName");
     subDir.create().then((ignore) {
       // Construct a long string of the form
       // 'tempdir/subdir/../subdir/../subdir'.
       var buffer = new StringBuffer();
       buffer.write(subDir.path);
       for (var i = 0; i < 1000; i++) {
         buffer.write("/../${subDirName}");
       }
       var long = new Directory("${buffer.toString()}");
       Expect.throws(() => long.listSync(),
                     (e) => e is DirectoryException);
       Expect.throws(() => long.listSync(recursive: true),
                     (e) => e is DirectoryException);
       d.deleteSync(recursive: true);
-      keepAlive.close();
+      asyncEnd();
     });
   });
 }
 
 void main() {
   testListNonExistent();
   testListTooLongName();
 }