The new version of Flow caught a type error here. memberOfContainingThread should be a boolean, but can be set to be nullable. This diff casts it to a boolean to avoid the type error.
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 D9859