[w3c/webcomponents] Returning a Proxy from a Custom Element constructor (#857)

Unfortunately, the following causes an error:

```js
class A extends HTMLElement {
  constructor() {
    super()
    return new Proxy(this, {
      get(target, key) {
        console.log('get property:' key)
        return target[key]
      }
    })
  }
}

window.customElements.define('a-element', A)
```

```html
<a-element></a-element>
```

```
Uncaught DOMException: custom element constructors must call super() first and must not return a different object
```

Can Custom Elements API be upgraded to support Proxied instances?

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