Include old/new values in $firebaseArray::$watch method.

oldnew.js

angular.factory('Arr', function($firebaseArray) {
  function Arr(ref) {
    this.oldVals = {};
    return $firebaseArray.call(this, ref);
  }
  
  Arr.prototype.$$process = function(event, rec, prevKey) {
    $firebaseArray.prototype.$$process.apply(this, arguments);
    this.oldVals[rec.$id] = rec;
  };
  
  Arr.prototype.$$getOldVal = function(key) {
    return this.oldVals.hasOwnProperty(key)? this.oldVals[key] : null;
  };

  Arr.prototype.$$notify = function(event, key, prevChild) {
    var eventData = {event: event, key: key, oldVal: this.$$getOldVal(key), newVal: this.$getRecord(key)};
    if( angular.isDefined(prevChild) ) {
      eventData.prevChild = prevChild;
    }
    angular.forEach(this._observers, function(parts) {
      parts[0].call(parts[1], eventData);
    });
  };
  
  return $firebaseArray.$extend(Arr);
});