Add an OutputStream.closed getter.

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 118 matching lines @@
     }
   }
 
   void destroy() {
     _socket.onWrite = null;
     _pendingWrites.clear();
     _socket.close();
     _closed = true;
   }
 
+  bool get closed() => _closed;
+
   void set onNoPendingWrites(void callback()) {
     _onNoPendingWrites = callback;
     if (_onNoPendingWrites != null) {
       _socket._onWrite = _onWrite;
     }
   }
 
   void set onClosed(void callback()) {
     _onClosed = callback;
   }
@@ skipping 63 matching lines @@
     }
   }
 
   Socket _socket;
   _BufferList _pendingWrites;
   Function _onNoPendingWrites;
   Function _onClosed;
   bool _closing = false;
   bool _closed = false;
 }