Re: [webcomponents] Change registries to be per document and never shared between documents (#369)

I think I prefer 1. And we later figure out how you get custom elements to instantiate in documents not associated with a window object (if ever, obviously that's not a good idea for XMLHttpRequest's document, unless you can only do this action after you get a hold of the document).

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https://github.com/w3c/webcomponents/issues/369#issuecomment-191760232