Issue #1987606 by vijaycs85, mparker17, InternetDevels, rdickert: Convert ajax_test_dialog() to a new style controller.
Issue #2177645 by joelpittet, InternetDevels: Replace theme() with drupal_render() in common.inc.
Issue #2177651 by joelpittet, InternetDevels: Replace theme() with drupal_render() in errors.inc.
Issue #2177665 by InternetDevels: Replace theme() with drupal_render() in theme.maintenance.inc.
Issue #2152213 by steveoliver, joelpittet, hussainweb, shanethehat, jenlampton, kpa, AnythonyR, EVIIILJ, kgoel, Cottser, dsdeiz, hanpersand: Convert theme_form_element() to Twig