Add case to please the analyzer

sdk/lib/io/websocket_impl.dart

 // Copyright (c) 2013, 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.
 
 part of dart.io;
 
 const String _webSocketGUID = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11";
 
 class _WebSocketMessageType {
   static const int NONE = 0;
@@ skipping 848 matching lines @@
 
   Future close([int code, String reason]) {
     if (!_writeClosed) {
       if (_isReservedStatusCode(code)) {
         throw new WebSocketException("Reserved status code $code");
       }
       _outCloseCode = code;
       _outCloseReason = reason;
       _writeClosed = true;
     }
-    if (!_sink._isBound) _sink.close();
+    if (!(_sink as _StreamSinkImpl)._isBound) _sink.close();
     return _sink.done;
   }
 
   static bool _isReservedStatusCode(int code) {
     return code != null &&
            (code < WebSocketStatus.NORMAL_CLOSURE ||
             code == WebSocketStatus.RESERVED_1004 ||
             code == WebSocketStatus.NO_STATUS_RECEIVED ||
             code == WebSocketStatus.ABNORMAL_CLOSURE ||
             (code > WebSocketStatus.INTERNAL_SERVER_ERROR &&
              code < WebSocketStatus.RESERVED_1015) ||
             (code >= WebSocketStatus.RESERVED_1015 &&
              code < 3000));
   }
 }