Issue #2551335 by alexpott, Wim Leers: Remove unnecessary SafeMarkup usage in tests and clear the static cache before every run

Issue #2497697 by marvin_B8, Crell, cilefen, joshi.rohit100, dawehner: PSR-7 support for ControllerResolver

Issue #2457887 by prateekMehta, stefan.r, rpayanm, alexpott: Use Utility\SafeMarkup class instead of Utility\String for placeholder(), checkPlain(),format() functions

Issue #2289917 by Wim Leers, mdrummond, lauriii, Manuel Garcia, emma.maria, Scionar, davidhernandez: Convert "messages" page element into blocks

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 #2281619 by dawehner, tim.plunkett: Convert most direct usages within module code of routing related request attributes to use RouteMatchInterface instead

Issue #2271251 by ParisLiakos, undertext, JeroenT, StevenPatz, tim.plunkett: Remove usages of watchdog() from non-form and non-controllers.

Issue #697760 by sun: Replace getInfo() in tests with native phpDoc + annotations (following PHPUnit).

Issue #2165475 by damiankloip, dawehner, martin107, tim.plunkett, jibran, xjm, Jalandhar: Provide a generic class resolver.

Issue #2079797 by ParisLiakos, Xano, amateescu, tim.plunkett: Provide a trait for $this->t() and $this->formatPlural().

Issue #2120235 by dawehner, YesCT: Regression: routing / tabs / actions / contextual links lack way to attach replacement arguments to UI strings.

Issue #2042739 by jhedstrom, bdone, dawehner, tim.plunkett: Convert system module's ControllerResolverTest to phpunit.

Issue #2068471 by dawehner, Crell, tim.plunkett, jibran, fubhy, larowlan: Normalize Controller/View-listener behavior with a Page object.

Issue #2119237 by cosmicdreams, Crell, damiankloip: Use \Symfony\Component\Debug\Exception\FlattenException instead of \Symfony\Component\HttpKernel\Exception\FlattenException.

Issue #2114069 by dawehner, Gábor Hojtsy, ursula: Fixed Routing / tabs / actions lack way to attach context to UI strings.

Issue #2031487 by fubhy, dawehner, pwolanin:  When replacing the upcasted values in the request attributes array, retain the original raw value too.