Implement async file API on top of isolates.

runtime/bin/directory_impl.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.
 
 
 class _DirectoryListingIsolate extends Isolate {
 
   _DirectoryListingIsolate() : super.heavy();
 
   void main() {
     port.receive((message, replyTo) {
       bool started = _list(message['dir'],
                            message['recursive'],
                            message['dirPort'],
                            message['filePort'],
                            message['donePort'],
                            message['errorPort']);
       replyTo.send(started);
+      port.close();
     });
   }
 
   bool _list(String dir,
              bool recursive,
              SendPort dirPort,
              SendPort filePort,
              SendPort donePort,
              SendPort errorPort) native "Directory_List";
 }
@@ skipping 118 matching lines @@
   bool _create(String path) native "Directory_Create";
   bool _delete(String path) native "Directory_Delete";
 
   var _dirHandler;
   var _fileHandler;
   var _doneHandler;
   var _errorHandler;
 
   String _path;
 }