Add method flush to output stream

runtime/bin/http_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 _HttpHeaders implements HttpHeaders {
   _HttpHeaders() : _headers = new Map<String, List<String>>();
 
   List<String> operator[](String name) {
     name = name.toLowerCase();
     return _headers[name];
@@ skipping 562 matching lines @@
   _HttpOutputStream(_HttpRequestResponseBase this._requestOrResponse);
 
   bool write(List<int> buffer, [bool copyBuffer = true]) {
     return _requestOrResponse._streamWrite(buffer, copyBuffer);
   }
 
   bool writeFrom(List<int> buffer, [int offset = 0, int len]) {
     return _requestOrResponse._streamWriteFrom(buffer, offset, len);
   }
 
+  void flush() {
+    // Nothing to do on a HTTP output stream.
+  }
+
   void close() {
     _requestOrResponse._streamClose();
   }
 
   void destroy() {
     throw "Not implemented";
   }
 
   void set onNoPendingWrites(void callback()) {
     _requestOrResponse._streamSetNoPendingWriteHandler(callback);
@@ skipping 737 matching lines @@
     sockets.addFirst(socketConn);
     socketConn._markReturned();
   }
 
   Function _onOpen;
   Map<String, Queue<_SocketConnection>> _openSockets;
   Set<_SocketConnection> _activeSockets;
   Timer _evictionTimer;
   bool _shutdown;  // Has this HTTP client been shutdown?
 }