With a common bytecode there would probably be a chance for languages to be fast and not stuck with translations to JavaScript for the next decade.
Citation extremely needed here. Bytecode isn't a magic speed potion — the implementation is what will determine how fast it runs. For example, V8 ROFLstomps over the JVM-based Rhino JavaScript implementation.
At any rate, any bytecode the JavaScript VM people agreed to would almost certainly correspond very closely to JavaScript, so it's not clear that a standard bytecode would make alternative languages much faster. On top of that, the optimizers in current JavaScript implementations are highly tuned for JavaScript, so a bytecode-compiled program running on V8 might actually be slower than the JavaScript we have today.
Basically, without an implementation, this all sounds like pie-in-the-sky talk to me.
Source maps are relatively easy: There's a public standard, a couple of browser makers have agreed to it, and it doesn't need universal acceptance since it's only for development. Basically, it doesn't require a lot of coordination.
A universal bytecode is almost the exact opposite — easy to describe in the abstract, but a coordination nightmare. I've never heard an actionable plan for making it happen. Just getting the community to agree on a standard bytecode would be a Herculean task†, and then you have to get Apple, Google, Mozilla and Microsoft to all say, "Yes, I'm willing to chain my VM to this bytecode at significant engineering expense."
Basically, feel free to work on this if you want, but you can't expect everybody else to hold their breath until it happens.
† You might think that "Herculean task" is an exaggeration, but just look at how much discussion and compromise was needed to get agreement on stabby function syntax for Harmony. And still people kvetched even once there was "agreement"!