Based on facebook#20932

Co-Authored-By: Dan Abramov <dan.abramov@gmail.com>

Alternate fix to facebook#20942

There was already a TODO to make this change, but at the time I left it,
I couldn't think of a way that it would actually cause a bug, and I was
hesistant to change something without fully understanding the
ramifications. This was during a time when we were hunting down a
different bug, so we were especially risk averse.

What I should have done in retrospect is put the change behind a flag
and tried rolling it out once the other bug had been flushed out.

OTOH, now we have a regression test, which wouldn't have otherwise, and
the bug it caused rarely fired in production.