code: function Cursor(properties) {
                      this.values = [];
                      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 Cursor.
  @memberof google.firestore.v1
  @interface ICursor
  @property {Array.<google.firestore.v1.IValue>|null} [values] Cursor values
  @property {boolean|null} [before] Cursor before
   
  Constructs a new Cursor.
  @memberof google.firestore.v1
  @classdesc Represents a Cursor.
  @implements ICursor
  @constructor
  @param {google.firestore.v1.ICursor=} [properties] Properties to set