Fix an analyzer error in http_parser's WebSocket implementation.

pkg/http_parser/lib/src/web_socket.dart

 // Copyright (c) 2014, 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.
 
 library http_parser.web_socket;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:math';
 import 'dart:typed_data';
@@ skipping 68 matching lines @@
   /// default); if it's a client, [serverSide] should be `false`.
   ///
   /// [WebSocket handshake]: https://tools.ietf.org/html/rfc6455#section-4
   factory CompatibleWebSocket(Stream<List<int>> stream,
         {StreamSink<List<int>> sink, bool serverSide: true}) {
     if (sink == null) {
       if (stream is! StreamSink) {
         throw new ArgumentError("If stream isn't also a StreamSink, sink must "
             "be passed explicitly.");
       }
-      sink = stream;
+      sink = stream as StreamSink;
     }
 
     return new _WebSocketImpl._fromSocket(stream, sink, serverSide);
   }
 
   /// Closes the web socket connection.
   ///
   /// [closeCode] and [closeReason] are the [close code][] and [reason][] sent
   /// to the remote peer, respectively. If they are omitted, the peer will see
   /// a "no status received" code with no reason.
@@ skipping 827 matching lines @@
             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));
   }
 }