Emit unhandled rejection event from HTML Spec

[ForbesLindesay]

see:

• https://tc39.github.io/ecma262/2016/#sec-host-promise-rejection-tracker
• https://html.spec.whatwg.org/#unhandled-promise-rejections

[edef1c]

Let event be a new trusted PromiseRejectionEvent

We can't do that, can we?

[ForbesLindesay]

We can't actually make it "trusted" but we can make an object that implements approximately the same interface (such that it would only be distinguishable by someone intentionally checking to make sure it wan't a polyfill).