Re: [dom] Let DOMTokenList.prototype.add() return false if there are no supported tokens (#111)

My problem with returning false if no supported tokens is present is that `add()` will always return false for things like `classList` and will return false for `<a rel>`'s `relList` on every value.

What would happen if one day we'd want to add supported tokens to `<a rel>`? There would be no way of telling if an attribute is really *not* supported. (whereas as defined today you can tell, even if in a cumbersome way)

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