constructor(childIter, mapFn) {
    /** @private @const {!Iterator<FROM>} */
    this.childIterator_ = getIterator(childIter);

    /** @private @const {function(FROM): TO} */
    this.mapFn_ = mapFn;
  }
@param {!Iterable<FROM>} childIter
@param {function(FROM): TO} mapFn