Add writeString method to OutputStream

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 65 matching lines @@
       _clientCloseHandler();
     }
   }
 
   Socket _socket;
   Function _clientCloseHandler;
   bool _closed = false;
 }
 
 
-class _SocketOutputStream implements SocketOutputStream {
+class _SocketOutputStream
+    extends _BaseOutputStream implements SocketOutputStream {
   _SocketOutputStream(Socket socket)
       : _socket = socket, _pendingWrites = new _BufferList();
 
   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);
@@ skipping 87 matching lines @@
     if (_streamErrorHandler != null) _streamErrorHandler();
   }
 
   Socket _socket;
   _BufferList _pendingWrites;
   var _onNoPendingWrites;
   var _streamErrorHandler;
   bool _closing = false;
   bool _closed = false;
 }