Since Web::Bindings::WindowObject inherits from JS::GlobalObject, it
cannot also inherit from Web::Bindings::EventTargetWrapper.
However, that's not actually necessary. Instead, we simply set the
Window object's prototype to the EventTargetPrototype, and add a little
extra branch in the impl_from() function that turns the JS "this" value
into a DOM::EventTarget*.
With this, you can now call window.addEventListener()! Very cool :^)
Fixes#4758.
The generic is<T>() uses dynamic_cast which is fine in the majority
of cases, but when one of them shows up in profiles, we can make it
faster by answering the is-a question manually.
