EventTarget constructor
This feature allows authors to create their own instances of EventTarget. Such instances do not participate in a tree.
Spec: https://dom.spec.whatwg.org/#dom-eventtarget-eventtarget
Tests:
- http://w3c-test.org/dom/events/EventTarget-constructible.any.html
- http://w3c-test.org/dom/events/EventTarget-constructible.any.worker.html
Spec discussion:
- https://github.com/whatwg/dom/issues/441
- https://github.com/whatwg/dom/pull/467