Make EventListener covariant

[creeefs]

Fixes #28357

Currently, the EventListener interface is invariant. As a result, functions that pass a subtype of Event fail to compile (e.g. see the CustomEvent and MessageEvent examples below).

CustomEvent handler

obj.addEventListener("customEvt", (e: CustomEvent) => {});

Type '(e: CustomEvent<any>) => void' is not assignable to type 'EventListener'.
    Types of parameters 'e' and 'evt' are incompatible.
        Type 'Event' is missing the following properties from type 'CustomEvent<any>': detail, initCustomEvent  TS2345

MessageEvent handler

eventSource.addEventListener("messageEvt", (e: MessageEvent) => {});

Type '(e: MessageEvent) => void' is not assignable to type 'EventListener'.
    Types of parameters 'e' and 'evt' are incompatible.
        Type 'Event' is missing the following properties from type 'MessageEvent': data, lastEventId, origin, ports, source  TS2345

As a result, it seems like a lot of people resort to using the as EventListener type assertion.

evtSource.addEventListener("messageEvt", ((e: MessageEvent) => {}) as EventListener);

Instead of having users resort to the as operator, I'm proposing we introduce a generic type variable that extends Event to the EventListener interface. We could then pass the appropriate sub-type to the addEventHandler functions for the respective objects (e.g. EventSource below).

interface EventSource extends EventTarget {
    addEventListener(type: string, listener: EventListenerOrEventListenerObject<MessageEvent>, options?: boolean | AddEventListenerOptions): void;
}

The net result would be to allow users to simply do:

evtSource.addEventListener("messageEvt", (e: MessageEvent) => {});

11/19/19: Note that the PR currently only implements the changes for the EventSource interface. I'd like to get some feedback before porting the change to every other object.

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