[lit-next] List changes for lit-next (newly used DOM APIs, etc.)

[bicknellr]

lit-next will end up using some 'newer' APIs that might not already be implemented in the polyfills. We should take an inventory of these and implement them. Some of the ES specific ones will probably be implemented in Babel / Closure, so we might not want to provide our own polyfill for those.

Known so far:

• JS
  • Map (example), Set
  • WeakMap (example)
  • Symbol (example), getOwnPropertySymbols
  • String#endsWith (example)
  • Array#fill (example)
  • Array#from (example)
  • Object#is
• DOM
  • Element#append() (example, PR #389)
  • Element#remove() (example, PR #390)
  • Element#classList (example)
  • Element#getAttributeNames
  • EventTarget#addEventListener and #removeEventListener with options object parameter (example)
  • Text constructor (example)
• HTML
  • CustomElementRegistry#upgrade (already implemented)

[bicknellr]

Holding this open because this is a lot less obvious in the closed pile.