Add method flush to output stream

runtime/bin/socket_stream_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 _SocketInputStream implements SocketInputStream {
   _SocketInputStream(Socket socket) : _socket = socket {
     if (_socket._id == -1) _closed = true;
     _socket.onClosed = _onClosed;
   }
 
@@ skipping 90 matching lines @@
 
   bool write(List<int> buffer, [bool copyBuffer = true]) {
     return _write(buffer, 0, buffer.length, copyBuffer);
   }
 
   bool writeFrom(List<int> buffer, [int offset = 0, int len]) {
     return _write(
         buffer, offset, (len == null) ? buffer.length - offset : len, true);
   }
 
+  void flush() {
+    // Nothing to do on a socket output stream.
+  }
+
   void close() {
     if (_closing && _closed) return;
     if (!_pendingWrites.isEmpty()) {
       // Mark the socket for close when all data is written.
       _closing = true;
       _socket._onWrite = _onWrite;
     } else {
       // Close the socket for writing.
       _socket._closeWrite();
       _closed = true;
@@ skipping 87 matching lines @@
     }
   }
 
   Socket _socket;
   _BufferList _pendingWrites;
   Function _onNoPendingWrites;
   Function _onClosed;
   bool _closing = false;
   bool _closed = false;
 }