constructor(childIter, filterFn) {
    /** @private @const {!Iterator<T>} */
    this.childIter_ = getIterator(childIter);

    /** @private @const {function(T): boolean} */
    this.filterFn_ = filterFn;
  }
@param {!Iterable<T>} childIter
@param {function(T): boolean} filterFn