readyState values are not defined on EventSource instance

[kalitine]

CONNECTING, OPEN, CLOSED constants are not defined on the new EventSource(...) object.

EventSource.CONNECTING === 0  // true
EventSource.OPEN === 1  // true
EventSource.CLOSED === 2  // true

Which is what expected, but not the following:

let es = new EventSource(url)
es.CONNECTING === undefined  // true
es.OPEN === undefined  // true
es.CLOSED === undefined  // true

I have not found details about this in the specification, but in Firefox 50.1.0 and Chrome 57 it works. Comparing to WebSocket API, it also works the same way in the browsers.