DisambiguatePossibleEpochNanoseconds: GetOffsetNanosecondsFor called with ISO Date-Time Record

[anba]

DisambiguatePossibleEpochNanoseconds, steps 6-9:

6. Let before be the latest possible ISO Date-Time Record for which CompareISODateTime(before, isoDateTime) = -1 and ! GetPossibleEpochNanoseconds(timeZone, before) is not empty.
7. Let after be the earliest possible ISO Date-Time Record for which CompareISODateTime(after, isoDateTime) = 1 and ! GetPossibleEpochNanoseconds(timeZone, after) is not empty.
8. Let offsetBefore be GetOffsetNanosecondsFor(timeZone, before).
9. Let offsetAfter be GetOffsetNanosecondsFor(timeZone, after).

But GetOffsetNanosecondsFor expects a BigInt input.

[ptomato]

Thanks for catching! This is a regression from #2925 that fails a type assertion, so I'm going to assume it doesn't need consensus to correct it.