In D8507, I update createThread so that it no longer returns newThreadInfo anymore, following the deprecation of some old client codeVersions.
At the time I should've also updated this code. The Flow upgrade forced me to type it, which then uncovered an error when checking for newThreadInfo, since it was removed from the type in D8507.
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 D9855