Fix an analyzer error in http_parser's WebSocket implementation.

pkg/http_parser/lib/src/web_socket.dart

Index: pkg/http_parser/lib/src/web_socket.dart
diff --git a/pkg/http_parser/lib/src/web_socket.dart b/pkg/http_parser/lib/src/web_socket.dart
index 98603ceca19c5ef89800917a3254b5e25c8355ac..77c9d2b873a41e2c6875f866c95ba748ceae7acf 100644
--- a/pkg/http_parser/lib/src/web_socket.dart
+++ b/pkg/http_parser/lib/src/web_socket.dart
@@ -86,7 +86,7 @@ abstract class CompatibleWebSocket implements Stream, 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);