Created by: bardiharborow
Fixes #17924 (closed) and fixes #20740 (closed). We have two ESLint configurations, js/.eslintrc.json (ES6, used for js/src/), and js/tests/.eslintrc.json (ES5, used for js/tests/, grunt/, docs/assets/js/src, docs/assets/js/ie-emulation-modes-warning.js and docs/assets/js/ie10-viewport-bug-workaround.js). As with my other linting changes, we may wish to tighten this later.