Another round of minor fixes in the IO libraries.

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.
 
 
 // Used for holding error code and error message for failed OS system calls.
 class _OSStatus {
   int _errorCode;
   String _errorMessage;
 }
@@ skipping 21 matching lines @@
              SendPort dirPort,
              SendPort filePort,
              SendPort donePort,
              SendPort errorPort) native "Directory_List";
 }
 
 
 class _DirectoryOperation {
   abstract void execute(ReceivePort port);
 
-  SendPort set replyPort(SendPort port) {
+  void set replyPort(SendPort port) {
     _replyPort = port;
   }
 
   SendPort _replyPort;
 }
 
 
 class _DirExitOperation extends _DirectoryOperation {
   void execute(ReceivePort port) {
     port.close();
@@ skipping 309 matching lines @@
   var _doneHandler;
   var _existsHandler;
   var _createHandler;
   var _createTempHandler;
   var _deleteHandler;
   var _errorHandler;
 
   String _path;
   _DirectoryOperationScheduler _scheduler;
 }