Issue #1431658 by Albert Volkman, xjm, kid_icarus, ChinggizKhan, rgristroph, connork: Massive API docs cleanup for system tests

Issue #1778942 by yched, tstoeckler, andypost: Fixed Discovery::getDefinition() / getDefinitions() : inconsistent return values for 'no result.
git commit -am Issue

Issue #1611430 follow-up by Berdir: Move call to file_create_url() to later in the verbose mode process.

Issue #1717678 by andypost, Berdir, attiks: Fixed Entity::createDuplicate() does not account for uuid property.

Issue #1788462 by samhassell: Remove references to 'active store' in comments for configuration system.

Issue #1608842 by chx, alexpott, sun, effulgentsia, beejeebus, tim.plunkett et al: Replace list of bootstrap modules, enabled modules, enabled themes, and default theme with config.

Issue #1480854 by tim.plunkett, Noyz, Bojhan, yoroy, kika: Convert operation links to '#type' => 'operations'.

Issue #1620114 by Rob Loach, sun: Explicitly register \Drupal\Component and \Drupal\Core to increase class lookup performance.

Issue #1677830 by slashrsm: Improve docs for user and node hooks in regards to database transactions

Issue #1608878 follow-up by tim.plunkett: Move Dropbutton CSS to dropbutton.base.css from node.admin.css.

Issue #1785086 by Jose Reyero, Gábor Hojtsy, catch, loganfsmyth, nedjo, stella: Added Introduce a generic API for interface translation strings.

Issue #1642526 by andypost, Berdir, dixon_: Add upgrade path from for generating UUIDs for all core entities.

Issue #1805776 by psynaptic, Eric_A, rbayliss: Fixed Fatal error: Class 'Drupal\Core\Template\ArrayIterator' not found in core/lib/Drupal/Core/Template/Attribute.php.

Issue #1642062 by tim.plunkett, xjm, chx, merlinofchaos, damiankloip, dawehner, Berdir, aspilicious, Fabianx: Add TempStore for persistent, limited-term storage of non-cache data.

Issue #742344 by sun, pillarsdotnet, tarmstrong, neilnz, wamilton, lucascaro: Allow forms to set custom validation error messages on required fields.

Issue #1742438 by larowlan, Christian Biggins, kim.pepper, Everett Zufelt: Fixed Vertical tabs header and container rendered even when no vertical tabs.

Issue #1706878 by tim.plunkett, lazysoundsystem: Add WebTestBase::assertThemeOutput() to allow modules to test theme function output.

Issue #1760530 by jenlampton, Albert Volkman, crazyrohila, Risse, justafish, Fabianx: Move all template files provided by core modules and themes into a 'templates' directory inside that module directory.

Issue #1788588 by pcambra | cosmicdreams: Fixed Remove unused variables from SimpleTest's WebTestBase.