This issue was originally filed by @butlermatt

A recent update to the Element class made Element.queryAll() return an ElementList [1]. However DocumentFragment has not been updated to have it's version of queryAll() do the same. DocumentFragment.queryAll() currently returns a List<Element> as per the old way.

This causes warnings in the editor when using shadowRoot.queryAll(..)[3] when trying to modify classes as such:
shadowRoot.queryAll(..).classes.add('..');