dillon1000/react

Check in a forked version of object-assign only for UMD builds (#18180)

* Check in a forked version of object-assign

This one uses ES modules so that we can inline it into UMD builds.

We could wait for object-assign to make an ESM export but we're going to
remove this dependency and assume global polyfills in the next version
anyway. However, we'd have to figure out how to keep the copyright header
and it'll get counted in terms of byte size (even if other tooling removes
it).

A lot of headache when we have our own implementation anyway. So I'll just
use that.

Ours is not resilient to checking certain browser bugs but those browsers
are mostly unused anyway. (Even FB breaks on them presumably.)

We also don't need to be resilient to Symbols since the way React uses it
we shouldn't need to copy symbols

* Don't transpile Object.assign to object-assign in object-assign

The polyfill needs to be able to feature detect Object.assign.