Re: [webcomponents] defined promise (#427)

If we're going to do that, we should do it soon. It would also be a good place to potentially stuff my `document.upgradeElement` from https://github.com/w3c/webcomponents/issues/419#issuecomment-194640804 if we do want to expose that primitive.

An alternative is statics on HTMLElement. That might match reality better since these are not very document-scoped after all, as we discovered in https://github.com/w3c/webcomponents/issues/369.

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