The new version of Flow needs all function parameters to have typehints. To type this correctly I looked at the library definition for forwardRef here.
Flow 0.217 introduces a library type for this, but I had to type it manually here.
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 D9768