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

@rniwa I see that, like you said, the custom element state isn’t set to "custom" till upgrade here. Using that as the test does appear to prevent `someElement.attachInternals()`. But it looks like it would prevent attaching in the constructor, too.

In [Upgrade an element](https://html.spec.whatwg.org/multipage/custom-elements.html#upgrades), the state is set to “failed” at step 3 (weird, but the note explains it takes care of an early exit for reentrancy). At step 6 the constructor is added to the stack and at step 8, the constructor runs. It isn’t till step 10, though, that — provided everything went swell — it changes from “failed” to “custom”.

I think that `custom element state` doesn’t (currently) align with the point where attachment should become available (unless I’m missing something). The gist makes sense to me but maybe a distinct flag would be needed?

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