Issue #2429671 by Berdir, Wim Leers: "The website has encountered an error. Please try again later." page is cached

Issue #356399 by dawehner, beejeebus, catch, sidharrell, klausi, nlisgo, Josh Waihi, Berdir, martin107: Optimize the route rebuilding process to rebuild on write

Issue #2448847 by dawehner, arlinsandbulte: [regression] Themes unable to implement hook_theme_registry_alter()

Issue #2369981 by Cottser: Not found templates are displayed literally instead of throwing an Exception due to string loader

Issue #2227227 by mpdonadio, amateescu, yched, larowlan, swentel, tstoeckler, fago: FieldTypePluginManager cannot instantiate FieldType plugins, good thing TypedDataManager can instantiate just about anything

Issue #2339491 by corbacho, dawehner, larowlan, effulgentsia, webflo: Ajax requests on forms with files fail on IE 9

Issue #2426031 by almaudoh, cpj: Remove deprecated uses of SessionManager::isEnabled(), SessionManager::enable() and SessionManager::disable()

Issue #2229145 by znerol, neclimdul, larowlan, alexpott, joelpittet, almaudoh, tadityar: Register symfony session components in the DIC and inject the session service into the request object

Issue #2414255 by lauriii, Jeff Burnz, dawehner, davidhernandez: Subtheme template inheritance working in reverse order

Issue #2051467 by jhedstrom, dawehner, amateescu, Berdir, herom, Mile23: Expand and convert to phpunit tests for \Drupal\Component\Transliteration

Revert "Issue #2229145 by znerol, neclimdul, larowlan, joelpittet, almaudoh: Register symfony session components in the DIC and inject the session service into the request object"

This reverts commit b926f5d8.

Issue #2229145 by znerol, neclimdul, larowlan, joelpittet, almaudoh: Register symfony session components in the DIC and inject the session service into the request object

Issue #2419005 by dawehner, alexpott: Aggregator: Uncaught PHP Exception Drupal\Component\Plugin\Exception\PluginNotFoundException: "The "FeedTitle" plugin does not exist."

Issue #1959806 by amateescu, jibran, goldorak, Wim Leers, yched, dawehner: Provide a generic 'entity_autocomplete' Form API element

Issue #2050759 by tim.plunkett, fietserwin: Move drupal_chmod and other code in file.inc to a Drupal\Core\File\FileSystem class

Issue #2343043 by cilefen, hussainweb: valid_email_address() should use egulias/EmailValidator and become deprecated

Revert "Issue #2388749 by alexpott, adci_contributor, ParisLiakos: Register symfony's mime guessers if they are supported"

This reverts commit 4dae1d99.

Issue #2388749 by alexpott, adci_contributor, ParisLiakos: Register symfony's mime guessers if they are supported

Issue #2197029 by dawehner, larowlan, ParisLiakos: Allow to inject dependencies into validation constraints

Issue #2348679 by znerol, Wim Leers: Move the remaining procedural page cache code to the page cache stack middleware

Issue #2107243 by amateescu, jibran, larowlan, Xano: Decouple entity reference selection plugins from field definitions

Issue #2291449 by Cottser: Add Twig template inheritance based on the theme registry, enable adding Twig loaders

Issue #2368797 by Wim Leers, dawehner, rteijeiro: Optimize ajaxPageState to keep Drupal 8 sites fast on high-latency networks, prevent CSS/JS aggregation from taking down sites and use HTTP GET for AJAX requests

Issue #2350837 by dawehner, Berdir, larowlan, Wim Leers: Convert most usages of EntityInterface::getSystemPath() to use routes

Issue #2379741 by Wim Leers, damiankloip: Add Renderer::getCacheableRenderArray() to encapsulate which data is needed for caching a render array and have views use it

Issue #2040135 by Wim Leers, Berdir: Caches dependent on simple config are only invalidated on form submissions

Issue #1973618 by dawehner, donquixote, plach, Mile23: DIC: Lazy instantiation of service dependencies (ProxyManager for "proxy services")

Issue #2151469 by rpayanm, lokapujya, JeroenT, alansaviolobo, Richard Damon, pflame, ianthomas_uk: Clean-up usage of deprecated list_themes() and _system_rebuild_theme_data() in favor of theme_handler service.