Without turning it on yet, of course.
The reason why I didn't use the general PseudoElement mechanism is because this
pseudo is a bit of its own thing, and I found easier to make ::selectors know
about it (because you need to jump to the assigned slot) than the other way
around.
Also, we need to support ::slotted(..)::before and such, and supporting multiple
pseudo-elements like that breaks some other invariants around the SelectorMap,
and fixing those would require special-casing slotted a lot more in other parts
of the code.
Let me know if you think otherwise.
I also don't like much the boolean tuple return value, but I plan to do some
cleanup in the area in a bit, so it should go away soon, I'd hope.
This patch doesn't modify any of the code because making a few things pub. I
did this first to make the next patch easier to audit.
MozReview-Commit-ID: 7PYxoS5bVGN