This is sort of what Flutter claims to be.

Flutter is a GUI toolkit, it's a different use case entirely with some small overlap (flutter can run on the web or could be used to build the front-end for a new browser)

I think he's referring to the fact that Flutter was started as a project, by some of Google's browser and web standards people, to see what the could come up with if they designed a browser from scratch with the knowledge we have today.

This is talked about in a few videos and articles by it's creators.

WASM does seem to be on the road to eventually being a better JS.

I wish WASM was AST based instead of stack based architecture.

What does that statement mean? As far as i know stack VS AST is Apple VS. Oranges. If you meant a register based architecture that would make sense. I don't know is wasm code is verifiable, in which case it's trivial to rewrite it to a register like architecture (like dalvik does for jvm bytecode).