Revert "Issue #2012812 by thedavidmeister, pwieck: drupal_render() can't distinguish between empty strings from theme() from an implemented hook and no hook being matched."

This reverts commit 431dc2e3.

Issue #2012812 by thedavidmeister, pwieck: drupal_render() can't distinguish between empty strings from theme() from an implemented hook and no hook being matched.

Issue #1828536 by sun, thedavidmeister, benjf, derek.deraps, jenlampton, dcam, pwieck: Rename 'type' variable of theme_item_list() to 'list_type'.

Issue #1839468 by ParisLiakos, damiankloip, dawehner, jibran: Replace aggregator rss parsing with Zend Feed.

Issue #1668866 by ParisLiakos, aspilicious, tim.plunkett, pdrake, g.oechsler, dawehner, Berdir, corvus_ch, damiankloip, disasm, marcingy, neclimdul: Replace drupal_goto() with RedirectResponse.

Issue #1852454 by patrickd, xjm, YesCT, Berdir, amateescu, Pancho, djroshi, dealancer, Wim Leers, jessebeach: Restrict module and theme name lengths to 50 characters.

Issue #1939066 by joelpittet, chrisjlee, Cottser, splatio, jenlampton: Convert theme_breadcrumb() to Twig.

Issue #1947536 by andypost, donquixote, larowlan, splatio, kim.pepper, alexpott, Crell, tim.plunkett: Convert drupal_get_breadcrumb() and drupal_set_breadcrumb() to a service.

Issue #1982980 by ParisLiakos, larowlan, dawehner, tim.plunkett: Move entity_get_form() to Drupal\Core\Entity\EntityManager::getForm().

Issue #1939090 by 2ndmile, hefox, Cottser, shanethehat, steveoliver, jenlampton, mr.baileys, derheap: Convert theme_tablesort_indicator() to Twig.

Issue #1363112 by mparker17, mgifford, tlattimore, Jacine, dcmouyard, bowersox, ry5n, ahdiaz: Simplify names of 'element-x' helper classes.

Issue #1987262 by the_phi, clemens.tolboom, Damien Tournoud, bshaffer: [Installation Error] Symfony: "The service definition 'request' does not exist.".

Issue #2003568 by dawehner, jhedstrom: Convert tags,attributes, diff and url validation unit tests to phpunit.

Issue #1924436 by JohnAlbin, rteijeiro: Remove separate CSS_AGGREGATE_SYSTEM aggregate file and fix drupal_add_library() to aggregate CSS properly.

Issue #2019481 by nod_: JavaScript AJAX commands object is borked: it is shared among all Drupal.ajax instances, preventing scoped overrides.

Issue #2009584 by rteijeiro, ry5n: Fixed Allow double underscores to pass through drupal_clean_css_identifier() as per new CSS standards.

Issue #2004408 by YesCT, penyaskito: Allow modules to alter the result of EntityListController::getOperations.

Issue #1972514 by Wim Leers, effulgentsia, Gábor Hojtsy, ekl1773: Fixed Impossible to set attributes for all entities.

Issue #2012836 by tim.plunkett: Replace drupal_region_class() with direct calls to drupal_html_class().

Issue #1825332 by Berdir, ParisLiakos, andypost, dawehner, jrglasgow, aspilicious: Introduce an AccountInterface to represent the current user.