I love the idea of Flutter, but tying it to Dart just seems like a way to guarantee it'll have a long slog to getting larger-scale usage. It feels like Google had a language that had no users and had to use it for _something_.
Re: Sibling comment it really is an order of magnitude better than many other languages. It's worth giving a try. Null safety alone is worth it (although other languages like C# now support this too).