Skip SelectEventPlugin extraction if no listeners

[hellendag]

If there are no listeners for onSelect yet, do not extract events. This way we can avoid issues where listeners have been set up for some event dependencies for SelectEventPlugin, but not all.

For instance, if topMouseDown has been registered but not topMouseUp, event extraction will set the mouseDown flag to true but never unset it. This leads to bugs when onSelect is registered and should be firing during normal key behavior. Since no topMouseUp has yet occurred to unset the flag, onSelect fails to fire.

[sophiebits]

Tests are failing:

>> not ok 88 - browser/eventPlugins/__tests__/SelectEventPlugin-test SelectEventPlugin should extract if an `onSelect` listener is present.
# Expected null not to be null.
# TypeError: 'null' is not an object (evaluating 'mouseup.type') in http://127.0.0.1:9999/test/index.html (line 79)

They're also super noisy because of a recent PR – I'll see if we can fix that separately.

[hellendag]

Yeah, saw that. I'm fixing now.

[sophiebits]

lgtm – merge at will.