Array.prototype.contains

[C. Scott Ananian]

And don't forget the related `DOMTokenList`, which is in
Element.classList and thus used by everyone always.

The `contains` and `item` methods are in `DOMTokenList`, so they
probably shouldn't be removed from `DOMStringList`.  But I don't think
anyone was seriously proposing that, just that `DOMStringList` would
be a subclass of `Array`...  (and maybe `DOMTokenList` would also be
related to `Array`?)
 --scott