Currently there are two UI tests, which utilise the PhantomJS webdriver. This webdriver was discontinued before it supported es2015 (ES6). So currently these two UI tests prevent using any es2015 syntax in javascript files, as that will result in a failing test: Facebook\WebDriver\Exception\NoSuchElementException: {"errorMessage":"Unable to find element with css selector 'span.dropdown-toggle'". Which is a cumbersome way of saying that unsupported syntax is used, eg let instead of var. 

JS/UI tests for 4.0 will take on a different format anyway, mostlikely through node/package.json, so in order to not let those tests prevent us from moving forward, these should be removed.