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

@rniwa, good catch ... looks like this issue can be demonstrated in Chrome’s implementation currently like this:

```js
x = document.createElement('x-');

customElements.define('x-', class extends HTMLElement {
  constructor() {
    super().attachInternals();
  }
});

x.attachInternals(); // works
customElements.upgrade(x); // throws
```


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