function baseAggregator(collection, setter, iteratee, accumulator) {
  baseEach(collection, function(value, key, collection) {
    setter(accumulator, value, iteratee(value), collection);
  });
  return accumulator;
}
Aggregates elements of `collection` on `accumulator` with keys transformed
by `iteratee` and values set by `setter`.

@private
@param {Array|Object} collection The collection to iterate over.
@param {Function} setter The function to set `accumulator` values.
@param {Function} iteratee The iteratee to transform keys.
@param {Object} accumulator The initial aggregated object.
@returns {Function} Returns `accumulator`.