Adds a feature flag `enableNewScheduler` that toggles between two
implementations of ReactFiberScheduler. This will let us land changes in
master while preserving the ability to quickly rollback.
Ideally this will be a short-lived fork. Once we've tested the new
scheduler for a week or so without issues, we will get rid of it. Until
then, we'll need to maintain two parallel implementations and run tests
against both of them. We rarely land changes to ReactFiberScheduler, so
I don't expect this will be a huge burden.
This commit does not implement anything new. The flag is still off and
tests run against the existing implementation.
Use `yarn test-new-scheduler` to run tests against the new one.
b1a56abd6a