HTMLCollection is an awkward legacy interface from the DOM spec.
It provides a live view of a DOM subtree, with some kind of filtering
that determines which elements are part of the collection.
We now return HTMLCollection objects from these APIs:
- getElementsByClassName()
- getElementsByName()
- getElementsByTagName()
This initial implementation does not do any kind of caching, since that
is quite a tricky problem, and there will be plenty of time for tricky
problems later on when the engine is more mature.
