[w3c/webcomponents] HTML element constructors need to Get(newTarget, "Prototype") (#560)

The [current spec says](https://html.spec.whatwg.org/#html-element-constructors) HTMLElement constructor needs to use the [prototype](https://html.spec.whatwg.org/#concept-custom-element-definition-prototype) stored by `customElements.define`.

This is not the way regular constructor works in a `super()` call.  Furthermore, this semantics is really cumbersome to implement in WebKit / JSC due to the way our engine's abstractions are designed.

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