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Per the explainer <https://dvcs.w3.org/hg/webcomponents/raw-file/tip/explainer/index.html#using-custom-elements-in-script> you need a way to refer to the constructor generated by a HTMLElementElement. It is possible that the "constructor" attribute could do double duty for this, just like <input id="x" form="y"> produces document.querySelector('#x').form => [object HTMLFormElement] then HTMLElementElement.constructor could be typed to being a script object.
http://dvcs.w3.org/hg/webcomponents/rev/847712bf845e