I think I mentioned it in a previous comment but I wanted to reiterate that HTML Imports are not a requirement for loading custom elements or their dependencies. Trey Shugart’s response had a lot of interesting details of how they’re tackling this in Skate. Didn’t want that to get lost in the discussion :)
Also, for what it’s worth, I came across this post the other day on using Webpack to do code splitting for a PRPL PWA. Wanted to drop this into the comment in case it’s interesting for folks.