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
There is a lot of logic in intro.js; now we test four cases:
1. (implicitly, via QUnit tests) A real browser with window being the global
2. Browserify where there are both global & window variables.
3. Node with jsdom where window is passed manually to the jQuery factory.
4. Pure Node with incorrect window passed; jQuery should throw then.
Previously the second & fourth case was not tested and the third was tested
in a way that interfered with the main test environment.
We now also test if in the Browserify case we're not creating a jQuery global
by default.
Fixes gh-2181
Closes gh-2234
