dillon1000/react

subtreeFlag warning: Fix legacy suspense false positive (#21388)

Legacy Suspense is weird. We intentionally commit a suspended fiber in
an inconsistent state. If the fiber suspended before it mounted any
effects, then the fiber won't have a PassiveStatic effect flag, which
will trigger the "missing expected subtreeFlag" warning.

To avoid the false positive, we'd need to mark fibers that commit in an
incomplete state, somehow. For now I'll disable the warning in legacy
mode, with the assumption that most of the bugs that would trigger it
are either exclusive to concurrent mode or exist in both.