[Events] Nested discrete events across systems

If an event in the old system is dispatched synchronously within an
event from the new system, or vice versa, and the inner event is a
discrete update, React should not flush pending discrete updates before
firing the inner event's handlers, even if the outer event is not
discrete.

Another way of saying this is that nested events should never force
React to flush discrete updates.

Arguably, if the outer event is not a discrete event, then the inner
event _should_ flush the pending events. However, that would be a
breaking change. I would argue this isn't so bad, however, given that
nested events are pretty rare. They don't fit nicely into our event
model regardless, since we don't support nested React renders. In the
future we should consider warning when events are nested.