Remove ES6-ism in demo.js
Remove ES6-ism in demo.js

tzuhsiulin/prosemirror,mattberkowitz/prosemirror,jacwright/prosemirror,tzuhsiulin/prosemirror,adrianheine/prosemirror,salzhrani/prosemirror,adrianheine/prosemirror,kiejo/prosemirror,salzhrani/prosemirror,mattberkowitz/prosemirror,jacwright/prosemirror,salzhrani/prosemirror,kiejo/prosemirror,tzuhsiulin/prosemirror,kiejo/prosemirror,adrianheine/prosemirror