Re: [whatwg/dom] Add "disable shadow" flag check to Element.attachShadow() (#760)

Doesn't this fail to respect the disable-shadow flag for customized built-in elements on the safelist? E.g. `<h4 is="special-h4">` with

```js
customElements.define("special-h4", class extends HTMLHeadingElement {
  disabledFeatures = ['shadow'];
}, { extends: 'h4' });
```

I think we need to pass the node's `is value`. I will add a commit to do that, but we should also update the tests.

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