Re: [whatwg/dom] Improving ergonomics of events with Observable (#544)

@bzbarsky 

> There is some sort of queuing happening in an Observable that is not being observed, right? Is this limited in any way? If I do on("mousemove") and then never subscribe to it, have I just created an unbounded memory leak?

No, there is no queuing at all. If you don't subscribe to the observable returned by `on('mousemove')` it will effectively be the same thing as not adding an event listener in the first place. Observables do nothing until they are subscribed to.

> When do the observables created by on get created? For example, if I on("DOMContentLoaded") after that event has fired, will I get that notification?

The observable itself is created at the moment `on("DOMContentLoaded")` is called. However observation does not start until you subscribe to the observable.

> Does on create a new observable each time?

Yes... however since whatever you're observing already exists, every one of them will effectively multicast. Technically, we *could* have each call to `on` return the same observable, as there wouldn't be any difference in behavior.

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