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

Yeah, but you can't register custom text nodes in a variety of documents. It's always unambiguous when you do `new someWindow.Text()` what document that will correspond to. If you do `someWindow.defineElement("x-tag", XTag)` and then `new otherWindow.XTag()` we have a IMO fatal ambiguity. It needs to be mandatory to clear up that ambiguity (2), or we need to eliminate the ambiguity (1).

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