dillon1000/react

Fix priority inference of next level of work (#15478)

Bugfix for `inferPriorityFromExpirationTime` function. It happened to
work in our existing tests because we use virtual time.

Flow would have caught this if expiration times were an opaque type. We
should consider that in the future. (The downside of opaque types is
that all operations would have to go through helper functions, which may
or may not get inlined by Closure.)