The point is no one uses plain javascript anymore so if you're going to use some domain specific language with its own build pipeline to generate compiled assets for the browser then Dart/Flutter is no worse than any other javascript framework.
Dart is very easy to learn so whether a lot of people know it or not is not really an issue. Any competent programmer will be able to learn it in a few days.
In my brief experience, learning the language and framework only took a few days and it was much easier than making sense of all the issues with the current frontend web stack. Dart also has JavaScript interop so using libraries written in js is not an issue. [1]