Yeah, that's what I wanted to say by the "tolerable" part. I know arrays, as well as other plain objects in JS, are capable of having arbitrary properties. But hey, how many libraries or code rely on that specific behavior of Array? For the "Greater Good", they can suppress those minor cases and simply extend the magic to the negative integer range. I believe this change can be landed without too much compatibility pain.
Putting arbitrary properties on an Array, in practice, turns it into a hashtable or a 'mixed' hashtable+Array (this depends on the JS implementation.
Dense array layouts are largely an optimization that the JS runtime applies if it's able to. So you 'can' put arbitrary properties on an Array, but you risk throwing away the benefits you get from using it instead of Object.
Sadly the risk of compatibility pain is huge for almost every change you can possibly imagine. I've been participating on es-discuss for a couple years now and tiny, harmless changes end up breaking real web applications on a regular basis - causing the spec committee to have to scratch their heads and figure out a fix. Sometimes there isn't a fix, and they have to rethink.
