Issue #1987822 by vijaycs85, jrglasgow, tim.plunkett: Convert system_date_time_lookup() to a new style controller.

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

Issue #1777956 by fago, klausi, sun, dasjo: Provide a way to define default values for entity fields.

Issue #1983548 by Berdir, LukyLuke_ch, giammi, effulgentsia: Convert contact message entities to the new Entity Field API.

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

Issue #1987066 by Kevin Morse, JohnAlbin, rteijeiro, echoz, ry5n: Rename files to match CSS file naming convention.

Issue #1987824 by nick_schuch, vijaycs85, ParisLiakos: Convert system_php() to a new style controller.

Issue #1957154 by joates, ayelet_Cr, xjm | Berdir: Replace calls to drupal_container()->get('module_handler()') service with Drupal::moduleHandler().

This reverts commit c4e52bb6.

Issue #1806478 by Cottser, joelpittet, kevinquillen: Make twig the default engine once all modules templates are converted from .tpl.php to .html.twig.

Issue #1980338 by vineet.osscube, nick_schuch: Fixed Fatal error: Class 'plugin.manager.block' not found in /core/modules/system/system.module on line 1162.

Issue #1987850 by oenie, vijaycs85, cosmicdreams, amateescu: Convert system_theme_disable() to a new style controller.

Issue #1620010 by dawehner, ParisLiakos, plach, jibran: Move LANGUAGE constants to the Language class.

Issue #1898034 by Cottser, jenlampton, joelpittet, Shawn DeArmond, idflood, Hydra, artofeclipse, rich.yumul, chrisjlee, gnuget, c4rl, thund3rbox: block.module - Convert PHPTemplate templates to Twig.

Issue #1898454 by c4rl, gnuget, Hydra, mr.baileys, Sean Charles, joelpittet: system.module - Convert PHPTemplate templates to Twig.

Issue #1946454 by andypost, kim.pepper, tim.plunkett, ParisLiakos: Convert all confirm_form() in system.module and system.admin.inc to the new form interface and convert route.

Issue #1925660 by ACF, tim.plunkett, mtift, heddn, Cottser: Convert system's system_config_form() to SystemConfigFormBase.

Issue #1974580 by nod_, Wim Leers: Introduce domReady to remove jQuery dependency from drupal.js and clean it up.

Issue #1927608 by EclipseGc, tim.plunkett, effulgentsia: Remove the tight coupling between Block Plugins and Block Entities.