EventSource events are always provided a MessageEvent

[Zirak]

Right now, EventSource.addEventListener has the following eventName => eventType mapping:

interface EventSourceEventMap {
    "error": Event;
    "message": MessageEvent;
    "open": Event;
}

And all other events are given the Event type. However, any event can be given a MessageEvent, e.g. given the following EventSource response:

event: foobar
data: I am foo

event: open
data: Why hello there

event: error
data: Now isn't this fun

And the following code:

var source = new EventSource('https://wherever');
source.addEventListener('foobar', console.log);
source.addEventListener('open', console.log);
source.addEventListener('error', console.log);

We'll get the following console output:

open { target: EventSource, isTrusted: true, srcElement: EventSource, … }

foobar { target: EventSource, isTrusted: true, data: "I am foo", … }

open { target: EventSource, isTrusted: true, data: "Why hello there", … }

error { target: EventSource, isTrusted: true, data: "Now isn't this fun", … }

The trivial solution would be to have the signature addEventListener(type: string, listener: (this: EventSource, ev: MessageEvent) => any, ...)

(omitted for brevity that the listener can also be an EventListenerObject receiving a MessageEvent).

I can send a PR if this seems like the right solution to you, I'm however a ts novice so maybe I'm missing a better solution?

Thanks.