Remove object-assign polyfill (#23351)

* Remove object-assign polyfill

We really rely on a more modern environment where this is typically
polyfilled anyway and we don't officially support IE with more extensive
polyfilling anyway. So all environments should have the native version
by now.

* Use shared/assign instead of Object.assign in code

This is so that we have one cached local instance in the bundle.

Ideally we should have a compile do this for us but we already follow
this pattern with hasOwnProperty, isArray, Object.is etc.

* Transform Object.assign to now use shared/assign

We need this to use the shared instance when Object.spread is used.