I don’t see how webhooks would be related or useful at all to a Flutter frontend.
It seems like what OP is looking for is a Dart implementation of the MessageBus client… which would probably have to be reverse engineered from this Javascript code. Another alternative might be to run the Javascript code within Dart, and then wrap it in bindings to translate from Javascript to Dart. A direct port to Dart should be the more performant of these two.
Someone please correct me if I’m wrong. This isn’t something I’ve dug into, but I think the idea of a Flutter frontend is intriguing.