Appears to be related to removing a bunch of Discourse constants for es6 compatibility, referenced here.
Also, neither of those bits of code appear to be in the master branch of the plugin, and retort appears to work fine on the latest Discourse (I just upgraded discourse-babble.com, which is also running retort, to latest), so I’m not sure how to help here.
That first line in particular looks mighty suspicious; Discourse master branch has this:
import { emojiExists } from 'pretty-text/emoji';
// scrub groups
groups.forEach(group => {
group.icons = group.icons.reject(obj => !emojiExists(obj));
});