Fix unresolved classes issues in dart:io.

sdk/lib/io/io.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.
 
 /**
  * The IO library is used for Dart server applications,
  * which run on a stand-alone Dart VM from the command line.
  * *This library does not work in browser based applications.*
  *
  * This library allows you to work with files, directories,
@@ skipping 11 matching lines @@
 #source('base64.dart');
 #source('buffer_list.dart');
 #source('chunked_stream.dart');
 #source('common.dart');
 #source('directory.dart');
 #source('directory_impl.dart');
 #source('eventhandler.dart');
 #source('file.dart');
 #source('file_impl.dart');
 #source('http.dart');
+#source('http_headers.dart');
 #source('http_impl.dart');
 #source('http_parser.dart');
 #source('http_session.dart');
 #source('http_utils.dart');
 #source('input_stream.dart');
 #source('list_stream.dart');
 #source('list_stream_impl.dart');
 #source('mime_multipart_parser.dart');
 #source('output_stream.dart');
 #source('path.dart');
 #source('path_impl.dart');
 #source('platform.dart');
 #source('platform_impl.dart');
 #source('process.dart');
 #source('socket.dart');
 #source('socket_stream_impl.dart');
 #source('stdio.dart');
 #source('stream_util.dart');
 #source('string_stream.dart');
 #source('timer_impl.dart');
 #source('secure_socket.dart');
 #source('secure_server_socket.dart');
 #source('websocket.dart');
 #source('websocket_impl.dart');