Fix type warnings in dart:io library.

runtime/bin/file_impl.dart

 // Copyright (c) 2012, 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 _FileInputStream extends _BaseDataInputStream implements InputStream {
   _FileInputStream(String name) {
     _file = new File(name);
     _data = [];
     _position = 0;
     _file.errorHandler = (String s) {
@@ skipping 400 matching lines @@
     request[1] = _name;
     _fileService.call(request).receive((path, replyTo) {
       if (path != null) {
         if (_directoryHandler != null) _directoryHandler(new Directory(path));
       } else if (_errorHandler != null) {
         _errorHandler("Cannot get directory for: ${_name}");
       }
     });
   }
 
-  void directorySync() {
+  Directory directorySync() {
     if (_asyncUsed) {
       throw new FileIOException(
           "Mixed use of synchronous and asynchronous API");
     }
     if (!existsSync()) {
       throw new FileIOException("Cannot get directory for: $_name");
     }
     return new Directory(_FileUtils.directory(_name));
   }
 
@@ skipping 767 matching lines @@
   Function _readByteHandler;
   Function _readListHandler;
   Function _noPendingWriteHandler;
   Function _positionHandler;
   Function _setPositionHandler;
   Function _truncateHandler;
   Function _lengthHandler;
   Function _flushHandler;
   Function _errorHandler;
 }