Named access on the `Window` object

[mookums]

https://html.spec.whatwg.org/multipage/nav-history-apis.html#named-access-on-the-window-object

We need to support the ability to get elements directly using their ID. This is used in various Web Platform tests where we end up having something like:

<iframe id="i"></iframe>

<script>
    i.onload = () => { console.log("loaded") }
</script>

This currently causes us to get a ReferenceError for i not existing.