Re: [webcomponents] defined promise (#427)

FWIW we've done something [similar in Skate](https://github.com/skatejs/skatejs#ready-elementorelements-callback-). The major use-case for us being that custom element upgrading can be erratic depending on when definitions are loaded and when the parser encounters them (elaborate explanation in the link). If you have a parent that needs to interact with a child, then you need a way to know when the child is upgraded. 

The other major use-case is what @jakearchibald mentioned which seems a result of `:defined` only working for the selected custom element. It won't work in scenarios where you want to ensure its descendants are `:defined` prior to showing the ancestor (correct me if I am wrong).

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https://github.com/w3c/webcomponents/issues/427#issuecomment-194541905