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.
My point is the frontend web stack is already very complicated for no good reason so using Flutter/Dart instead of dealing with webpack or whatever the latest build tool is for frontend web projects isn't that big of a difference.
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.
And my point is that since you're going to learn a JS-like with build tools, you would better be served by JS or TS instead of Dart, since Dart isn't that popular, the frontend experience is terrible and there's not much choice in terms of backend and libraries.
I've tried js, ts, and dart and dart is a much more pleasant language with simpler tooling than the current js/ts web stack. We can continue going back and forth on this but I don't think we're getting anywhere. My suggestion was to someone that was looking to avoid the main hassles of web development and I suggested Flutter/Dart to simplify the toolchain and not worry about having to pick and choose frontend frameworks and build tools because all of that is baked into Flutter/Dart already.
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]
