dillon1000/react

[scheduler] 4/n Allow splitting out `schedule` in fb-www, prepare to fix polyfill issue internally (#12900)

* Use local references to global things inside 'scheduler'

**what is the change?:**
See title

**why make this change?:**
We want to avoid initially calling one version of an API and then later
accessing a polyfilled version.

**test plan:**
Run existing tests.

* Shim ReactScheduler for www

**what is the change?:**
In 'www' we want to reference the separate build of ReactScheduler,
which allows treating it as a separate module internally.

**why make this change?:**
We need to require the ReactScheduler before our rAF polyfill activates,
in order to customize which custom behaviors we want.

This is also a step towards being able to experiment with using it
outside of React.

**test plan:**
Ran tests, ran the build, and ran `test-build`.

* Generate a bundle for fb-www

**what is the change?:**
See title

**why make this change?:**
Splitting out the 'schedule' module allows us to load it before
polyfills kick in for rAF and other APIs.

And long term we want to split this into a separate module anyway, this
is a step towards that.

**test plan:**
I'll run the sync next week and verify that this all works. :)

* ran prettier

* fix rebase issues

* Change names of variables used for holding globals