lib/mixins/update.js
function _getChangedProperties() {
  var retVal = {}, key;
  for (key in this._changed) {
    retVal[key] = this._changed[key];
  }
  return retVal;
}
function _getChangedProperties() {
  var retVal = {}, key;
  for (key in this._changed) {
    retVal[key] = this._changed[key];
  }
  return retVal;
}
Get only changed properties as a hash.

@return {Object}
@api private