This is a really old utility. We only use it in web/calendar/day.react.js now.
Besides adding some types, I also had to add an invariant. The old code would've crashed in this case anyways (on the other invariant), but the second invariant was necessary for Flow.
NOTE: CI will fail on this diff. I considered the possibility of fixing Flow errors BEFORE upgrading Flow, but it wasn't possible... in some cases, the fixes to support the new version of Flow caused errors in the old version. I could have hidden these type errors with $FlowFixMe lines and then later revert those, but that seemed like too much busy work.
Depends on D9780