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

Which long term are you thinking of?  Note that one issue with the boolean setup that was raised is what happens if a list with no supported tokens changes to one with supported tokens.  Suddenly you have no way to trust return values until everyone updates, and you can't even do the `!== undefined` trick.  Hence the desire for a tristate return value to start with.

But also, what experience we have with the `!== undefined` thing shows that people won't get it right.  At least the one person I saw try to handle it so far got it wrong, and that was a browser engineer...

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