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
1. Use the short name of the preset in the config.
2. Run ESLint first on non-minified files.
3. Explicitly specify environments in every config file (those settings cascade
which means we've been assuming a Node.js environment where we shouldn't have).
