Moving the Web to Dart/Flutter would cost billions. How would it help in ANY way solving any challenge that currently exists? What even are these challenges? The OP is pretty blurry on that.
I forgot to add "not controlled by a single corporation" to my list of requirements. Also not gradually typed, I think? But I would consider it if it weren't all tied into Google and had a larger ecosystem.
Typescript is completely open source (Apache 2.0 licensed) and all of its roadmap, planning, issue tracking, peer reviewing, is also out in the open (in GitHub Issues). It's about as "controlled" by Microsoft as Linux is "controlled" by Red Hat at this point.
OK, but the point of comparison was Dart/Flutter, which I suppose is similarly open.
Open source does not mean that it's not controlled by a single person or organization. It's not meant as a critique though, as the alternative I guess is a language designed by committee, which has its own problems.