let's put an:
// XXX use mix.js when it lands in the streaming patch.
on this.  I have changes in my patch to do that, but it's actually a bit annoying because of the require.js map changes, so let's leave this like it is for now.  (And I'll change mix.js to use this fanciness.  Assuming there's no shape problems with propagating this way?  I assume there shouldn't be.)