Do not rely on window in _unsafeReadProtoTagged

[rnons]

Close #10

A reproducible (on Chrome) example is here https://codepen.io/rnons/pen/gObLQzY?editors=1011

<!-- index.html -->
<iframe></iframe>
<script>
const iframe = document.querySelector('iframe')
  iframe.contentWindow.addEventListener('click', ev => {
    console.log(ev.constructor.name)
    console.log(ev instanceof window.MouseEvent) // false!
  })
</script>

[garyb]

Maybe we could keep the early exit in the case of a positive result, but remove the early exit in the negative - the instanceof check should generally be faster than having to traverse the prototype chain.

[thomashoneyman]

@garyb Do you mean simply removing this line, so that we return early in case of a positive result, but fall through otherwise?

[garyb]

Yeah, exactly.