All the other files were already taken from the external directory.
The fact core-js was taken from node_modules broke IE core tests on TestSwarm.
Ref gh-4865
Ref gh-4870
(partially cherry picked from 345cd22e56)
So far, we've been testing that jQuery element iteration works with polyfilled
Symbol & transpiled for-of via a Node test with jsdom with the Symbol global
removed. Unfortunately, jsdom now requires Symbol to be present for its internal
functionality so such a test is no longer possible. Instead, it's been migrated
to an iframe test with transpiled JavaScript.
This PR also enables us to use ECMAScript 2017 or newer in Node.js code.
Closes gh-4305
