[ShadyDOM] HTMLElement ids don't get scoped

Unless I'm doing something wrong, it appears as if element ids aren't getting properly scoped inside their shadow root. If I have a custom input component with a label and input, the `htmlFor` and `id` need to be dynamically set (or have some unique ID appended to them) to keep the form accessible, the markup valid and maintain general HTML rules (clicking an label focuses the input, tooltips associated with `aria-describedby` etc.).

I had assumed that there would be some modification to the ID attribute to keep the HTML valid. Presumably, we could then patch `ShadowRoot.prototype.getElementById` to use some pre-configured `Map` object instead of the normal `getElementById`.