Re: [w3c/webcomponents] Scoped Custom Element Registries (#716)

to have a easy implamentation of this we would need 
```js
document.createElement('p',class extends ParagraphHTMLElement {})
document.createElement('my-element',class extends HTMLElement {})
```
note that we don't need to supply extends since we don't hornor the is attribute when we create the element our self its not the ui thats upgrading the element .

## Alternativ we could expose upgrade api.
```js
customElements.upgrade(el,class extends ParagraphHTMLElement {})
```

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