This task is about the maintenance scripts in MediaWiki core.
https://gerrit.wikimedia.org/r/#/c/378543/ broke the database dumps, because the autoloader is only invoked after the require_once RUN_MAINTENANCE_IF_MAIN thing. But there's a class in this file that extends DumpFilter *before* the autoloader. If those classes were also moved out of this file then it would all just work.
So we need to move the ExportProgressFilter class from that file into includes/export/ExportProgressFilter.php, and add it to autoload.php in MediaWiki core. Then https://gerrit.wikimedia.org/r/#/c/378543/ can be re-applied.