The promise would resolve immediately, nulling out `flushPromise`. This would
then immediately be set from `new Promise((resolve, reject) => {...})` turning
it back into non-null `flushPromise`. The resolve handler was called so the
next `flush()` would see "oh yes, there is a non-null `flushPromise`" then
promptly try to set `flushAgainPromise` which chains off the resolved
`flushPromise` which relied on `flushPromise` being `null`ed out after
`resolve()`, causing the chained `flush()` to see "oh yes, there is a non-null
`flushPromise`" which... ad infinitum.
This PR fixes it by making the nulling out asynchronous but the fact it took
me this long to debug this issue indicates to me that this is a terrible piece
of code. Will re-write.
Kegan Dougal
f3df86872b