Thanks to bug 1642505, our integration of ICU and tzdata and related imported code in the tree is now documented:
If you ever wondered how our imported ICU is integrated into SpiderMonkey and Gecko builds, this documentation should get you well on the way to understanding.
Perhaps most importantly: this document now enumerates all the relevant invocations required to *update* ICU, or tzdata, or integrations into the JS engine! So if anba and I happen not to be around at some time for some reason, someone else *should* be able to just read that document, follow the relevant instructions, and things should be good to go again. The bus factor is now at least somewhat greater than two!
Anyway: take a look, read through the thing, if anything doesn't make sense to you, file bugs! Thus far only the two of us who already knew this stuff have looked at it. (And having finally written all these steps down, I fully intend to forget everything in it. :-) ) The review of eyes that are not familiar with any of this are definitely appreciated.
And if you're feeling *really* motivated, you could maybe even convert it to reStructuredText and integrate it into intl/docs? ;-) I picked a format and wrote stuff, and I did not want to let particular format impede actual landing -- but now that it's landed, moving it is likely reasonable.
0. There *might* be small bits of futzing required to deal with lack of bug 1642176 being fixed, particularly if you're dealing with this on a branch. But those are niggling details, and you might have been able to figure them out even without this pointer, perhaps.