observer functions not firing because childChanged firebase event calls set on entire object.

[ryanwtyler]

https://github.com/GoogleWebComponents/firebase-element/blob/master/firebase-collection.html#L612

this line calls set on the entire object that changed. not the subproperty of the object that changed. so my dom-repeat sort function runs on the client that made the local change. but other clients don't get set called on the property that fires the sort function.