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

I've just posted a larger comment on [the 831 issue](https://github.com/whatwg/dom/issues/831), which should be read before this one for context. But I wanted to point out one problem with the `attachShadow({mode: 'closed', returnExistingDeclarativeShadowRoot: true});` version of this feature. That will not work for the case where there is an imperative/declarative race, and the imperative code "wins". In that case, the component will likely want to detect that the declarative content hasn't loaded, and wait until later to hydrate. But there will be no way to detect that situation, since calling `attachShadow()` will actually attach a shadow root and block the declarative content. If, on the other hand, we go with an `ElementInternals.shadowRoot` accessor, this situation can be properly detected no matter who wins the race.

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