This constructor has undergone a handful of editorial changes that we
fell behind on. But we weren't able to take the updates until now due to
a spec bug in those updates. See:
https://github.com/tc39/ecma402/commit/3f029b0
The result is that we can remove the inheritance of Intl::DateTimeFormat
from Unicode::DateTimeFormat; the former now contains the latter as an
internal slot.