Avoid a warning due to bug in Dart SDK

pkg/mdns/lib/src/native_protocol_client.dart

Index: pkg/mdns/lib/src/native_protocol_client.dart
diff --git a/pkg/mdns/lib/src/native_protocol_client.dart b/pkg/mdns/lib/src/native_protocol_client.dart
index ffbdd8f5317b25e8c3b7f8f28ce639d53480de79..187997a056fc327941ae8f7ca4afca8b69b514e9 100644
--- a/pkg/mdns/lib/src/native_protocol_client.dart
+++ b/pkg/mdns/lib/src/native_protocol_client.dart
@@ -101,7 +101,7 @@ class NativeProtocolMDnsClient implements MDnsClient {
       _sockets.add(socket);
 
       // Join multicast on this interface.
-      _incoming.joinMulticast(mDnsAddress, interface);
+      (_incoming as dynamic).joinMulticast(mDnsAddress, interface);
     }
     _incoming.listen(_handleIncoming);