code: function addCustomFeatureProperties(entity) {
    for (var i = 0; i < customProperties.length; i++) {
      if (entity.propertyNames.indexOf(customProperties[i]) === -1) {
        entity.addProperty(customProperties[i]);
      }
    }
  }
docs: Features have 'entityCollection', 'properties' and 'data' properties, which we must add to the entity's property list, if they're not already there. (In case they are added in a future version of Cesium.)