Use the web_socket_channel package.

lib/shelf_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.
 
-import 'package:http_parser/http_parser.dart';
 import 'package:shelf/shelf.dart';
+import 'package:web_socket_channel/web_socket_channel.dart';
 
 import 'src/web_socket_handler.dart';
 
 /// A typedef used to determine if a function takes two arguments or not.
 typedef _BinaryFunction(arg1, arg2);
 
 /// Creates a Shelf handler that upgrades HTTP requests to WebSocket
 /// connections.
 ///
 /// Only valid WebSocket upgrade requests are upgraded. If a request doesn't
@@ skipping 34 matching lines @@
       throw new ArgumentError("If protocols is non-null, onConnection must "
           "take two arguments, the WebSocket and the protocol.");
     }
 
     var innerOnConnection = onConnection;
     onConnection = (webSocket, _) => innerOnConnection(webSocket);
   }
 
   return new WebSocketHandler(onConnection, protocols, allowedOrigins).handle;
 }