Add IOSink:flush().

sdk/lib/io/http_impl.dart

 // Copyright (c) 2013, 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.
 
 part of dart.io;
 
 class _HttpIncoming extends Stream<List<int>> {
   final int _transferLength;
   final Completer _dataCompleter = new Completer();
   Stream<List<int>> _stream;
@@ skipping 450 matching lines @@
   }
 
   void addError(error, [StackTrace stackTrace]) {
     _dataSink.addError(error, stackTrace);
   }
 
   Future<T> addStream(Stream<List<int>> stream) {
     return _dataSink.addStream(stream);
   }
 
+  Future flush() {
+    return _dataSink.flush();
+  }
+
   Future close() {
     return _dataSink.close();
   }
 
   Future<T> get done => _dataSink.done;
 
   Future _writeHeaders({bool drainRequest: true}) {
     if (_headersWritten) return new Future.value();
     _headersWritten = true;
     headers._synchronize();  // Be sure the 'chunked' option is updated.
@@ skipping 1766 matching lines @@
 
   void addError(error, [StackTrace stackTrace]) =>
       _socket.addError(error, stackTrace);
 
   Future<Socket> addStream(Stream<List<int>> stream) {
     return _socket.addStream(stream);
   }
 
   void destroy() => _socket.destroy();
 
+  Future flush() => _socket.flush();
+
   Future close() => _socket.close();
 
   Future<Socket> get done => _socket.done;
 
   int get port => _socket.port;
 
   InternetAddress get address => _socket.address;
 
   String get remoteHost => _socket.remoteHost;
 
@@ skipping 246 matching lines @@
   final Uri location;
 }
 
 String _getHttpVersion() {
   var version = Platform.version;
   // Only include major and minor version numbers.
   int index = version.indexOf('.', version.indexOf('.') + 1);
   version = version.substring(0, index);
   return 'Dart/$version (dart:io)';
 }