Reland Remove redundant initial of isArray (#21188)

* Remove redundant initial of isArray (#21163)

* Reapply prettier

* Type the isArray function with refinement support

This ensures that an argument gets refined just like it does if isArray is
used directly.

I'm not sure how to express with just a direct reference so I added a
function wrapper and confirmed that this does get inlined properly by
closure compiler.

* A few more

* Rename unit test to internal

This is not testing a bundle.

Co-authored-by: Behnam Mohammadi <itten@live.com>