Exception on stopListening() of DOMEmitterMixin

[oskarwrobel]

How to reproduce:

const emitter = Object.create( DOMEmitterMixin );

emitter.listenTo( element, 'someEvent', callback );
emitter.stopListening( element, 'someEvent', callback );

emitter.stopListening(); // TypeError: Cannot read property 'removeListener' of undefined.

This works fine:

const emitter = Object.create( DOMEmitterMixin );

emitter.listenTo( element, 'someEvent', callback );
emitter.stopListening( element, 'someEvent' ); // Without callback

emitter.stopListening();

[Reinmar]

This may be needed for #114.

[oskarwrobel]

This is because parent EmitterMixin.off( event, callback ) removes only specified callback from callbacks array, and keeps event object in memory even if callbacks array is empty (is it expected?).

We can easily fix it by adding condition which prevent from removing non existing listener here or we can consider to improve EmitterMixin.off() to remove event from emitter when callbacks array is empty.