Re: [dom] Change DOMTokenList's add() return value. (#114)

@zcorpan - though, if A is chosen, the `DOMSettableTokenList` removal would be funny, because this introduces yet another subclass.
If we want a one-solution-fits-everything approach, then perhaps have `boolean DOMTokenList.prototype.supports(string value)` as well as `boolean DOMTokenList.prototype.hasSupportedTokens`.
The almighty `DOMTokenList`.

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