The [idl interface](https://dom.spec.whatwg.org/#interface-customevent) and servo's implementation had diverged.
* Extra arguments to initCustomEvent are optional (fixes `dom/events/CustomEvent.html`)
* The CustomEvent constructor is infallible
* "[Exposed=*]" (and the same for the "Event" interface since it's
CustomEvent's parent.)
Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>
Implement historical returnValue attribute for Event interface
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Implementation of `returnValue` attribute for Event interface.
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