code: function CollectionSelector(properties) {
                          if (properties)
                              for (var keys = Object.keys(properties), i = 0; i < keys.length; ++i)
                                  if (properties[keys[i]] != null)
                                      this[keys[i]] = properties[keys[i]];
                      }
docs: Properties of a CollectionSelector.
  @memberof google.firestore.v1.StructuredQuery
  @interface ICollectionSelector
  @property {string|null} [collectionId] CollectionSelector collectionId
  @property {boolean|null} [allDescendants] CollectionSelector allDescendants
   
  Constructs a new CollectionSelector.
  @memberof google.firestore.v1.StructuredQuery
  @classdesc Represents a CollectionSelector.
  @implements ICollectionSelector
  @constructor
  @param {google.firestore.v1.StructuredQuery.ICollectionSelector=} [properties] Properties to set