Use isolates for directory listing.

runtime/tests/dart/src/DirectoryTest.dart

 // Copyright (c) 2011, 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.
 
 class DirectoryTest {
   static void testListing() {
     bool listedSomething = false;
     Directory directory = new Directory.open(".");
@@ skipping 10 matching lines @@
       Expect.isTrue(completed, "directory listing did not complete");
       Expect.isTrue(listedSomething, "empty directory");
       directory.close();
     });
 
     directory.setDirErrorHandler((dir) {
       Expect.fail("error listing directory");
     });
 
     directory.list();
+
+    // Listing is asynchronous, so nothing should be listed at this
+    // point.
+    Expect.isFalse(listedSomething);
   }
 
   static void testMain() {
     testListing();
   }
 }
 
 main() {
   DirectoryTest.testMain();
 }