it's an IE shim for new  methods, Object.[keys,defineProperty, etc], Date.now, et al.
only IE would load the shim as its a sync external script tag in the head behind a conditional comment.
so, if you have a solid foundation, you can simply write the 21st-century version and not have to worry about old browsers; they will run the new code just like real browsers.
or you can write expressly for obsolete browsers and make it harder on yourself to save a few KB that only ancient devices will ever fetch anyway...