What you're missing is that there is no need to do that. Using JavaScript as an intermediary representation for other languages on a permanent basis is kind of ridiculous. Improving browser support for JavaScript-as-IR provides no benefit over adding support for a "real" IR and introduces superfluous complexity which is the enemy of both performance and security.

Compiling other languages to JavaScript is a tolerable transition mechanism while browsers that don't support something better are still popular, but trying to improve the ability of browsers to interpret compiled-to-JavaScript code from other languages is farcical -- if you're going to update the browser then update it to support something better than JavaScript, not to improve the performance of the horrible legacy transition kludge.