Add method flush to output stream

runtime/bin/output_stream.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.
 
 /**
  * Output streams are used to write data sequentially to a data
  * destination e.g. a connected socket or an open file.
  *
  * An output stream provides internal buffering of the data written
  * through all calls to [write] and [writeFrom] if data cannot be
@@ skipping 33 matching lines @@
    * Write a string to the stream using the given [encoding].The
    * default encoding is UTF-8 - [:Encoding.UTF_8:].
    *
    * Returns true if the data could be written to the underlying
    * communication channel immediately. Otherwise the data is buffered
    * by the output stream and will be sent as soon as possible.
    */
   bool writeString(String string, [Encoding encoding]);
 
   /**
+   * Flushes data from any internal buffers as soon as possible. Note
+   * that the actual meaning of calling [flush] will depend on the
+   * actual type of the underlying communication channel.
+   */
+  void flush();
+
+  /**
    * Indicate that all data has been written to the output
    * stream. When all data has been written to the communication
    * channel it will be closed.
    */
   void close();
 
   /**
    * Close the communication channel immediately ignoring any buffered
    * data.
    */
@@ skipping 14 matching lines @@
    */
   void set onClosed(void callback());
 
   /**
    * Sets the handler that gets called when the underlying
    * communication channel gets into some kind of error situation.
    */
   void set onError(void callback(e));
 }