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

OK I see what you are saying. I suppose we could make the classList case also be falsy, e.g. continue to return `undefined`. But I also don't expect people to check the return value for classList, and I don't expect classList to get a list of supported keywords. But certainly it is difficult to predict the future.

I would be fine with changing back classList to return `undefined`, and either keep the other values as boolean, or use "" and "supported".

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