Expression based on array does not update when array is modified

[stylpe]

See http://jsbin.com/wemunuzulonu/2/edit for several examples, type something in the input box and watch the behaviour.

[jmesserly]

related: #13

we don't observe arrays from polymer-expressions, but other parts of the system do. Maybe we should just fix it for consistency. Not sure the performance impact, though.

[sjmiles]

I've been advocating this for some time, but I've not worked on this module. I'm afraid that at the moment, this module is without an owner.

[jmesserly]

@sjmiles -- I'll have to play with it, but I suspect the fix is pretty simple, if we want to go ahead with it.
(it's not unlike what we do here in properties.js or here in templatebinding.js )