Re: [w3c/webcomponents] Expose shadowRoot on element internals (#871)

Regardless of the past discussion about `attachInternals`, it seems problematic that anyone can just call `attachInternals` and gain access to a closed shadow root. In particular, between the time a shadow root is attached and custom element's definition gets loaded (i.e. customElements.define is called), anyone can come in & call `attachInternals`.

Now that I'm thinking about this problem more, perhaps the easiest solution is to forbid ElementInternals to be created unless the underlying element's custom element state is "custom". This forces custom element's constructor to be the very first thing to have the ability to call `attachInternals`. 

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