`dirs` was a hash before this change. For example, see https://github.com/rails/rails/blob/master/activesupport/test/file_update_checker_test.rb#L104 or https://github.com/rails/rails/blob/master/activesupport/test/file_update_checker_test.rb#L91. I believe the key to the hash is the directory path and the value is the file extension to check in that directory.