Are the js folks cool with this?

I've asked a handful of JS developers about this IRL and they all say something like "as long as you make it easy to build SM standalone, we don't care." There was talk of creating a mach command to facilitate easier standalone SM builds. I /think/ bug 912359 tracks that. We may need to provide that as part of delivering this work so their build procedure doesn't get too complicated.

It would make porting (well, my porting work, at least) a lot easier if there were a simple way to build a js shell before building the rest of the browser.