Issue #3420986 by kim.pepper, sorlov, larowlan, mstrelan: Convert Condition plugin discovery to attributes

Issue #2273381 by clayfreeman, tim.plunkett, andypost, dawehner, EclipseGc, jungle, xjm, alexpott, Berdir: Convert ContextAwarePluginBase to traits

Issue #3065586 by Charlie ChX Negyesi, alexpott: ConditionManager::execute() implementation doesn't conform to the interface

Issue #2949177 by tim.plunkett, samuel.mortenson, mark_fullmer, tedbow, EclipseGc, andypost: Introduce a service that returns filtered plugin definitions

Issue #2776975 by joelpittet, dawehner, tim.plunkett, xjm, pfrenssen: March 3, 2017: Convert core to array syntax coding standards for Drupal 8.3.x RC phase

Issue #2665992 by alexpott, klausi, heykarthikwithu, xjm, tstoeckler: @file is not required for classes, interfaces and traits

Issue #2614604 by rakesh.gectcr, dawehner: PHPdoc is missing for an overridden function in class ConditionManager

(cherry picked from commit b33ed035)

Issue #2502621 by Pravin Ajaaz, hussainweb, Peacog, jhodgdon, ivanjaros: Replace implement notes with inheritdoc tag

Issue #2422815 by Xano, fgm, tim.plunkett, dawehner: Don't initialize the discovery object in plugin managers, unless needed

Issue #2349991 by fago, Xano, amateescu: Provide a trait for categorizing plugin managers and use it for conditions and actions

Issue #2318281 by tim.plunkett, dawehner | kim.pepper: Provide a standard way to check a plugin is an instance of the required interface.

Issue #2281635 by fubhy, er.pushpinderrana, EclipseGc, blueminds, tim.plunkett: \Drupal\Core\Plugin\Context\Context should use a ContextDefinition class instead of arrays.

Issue #2277981 by tim.plunkett, EclipseGc, fago: Provide a service for handling context-aware plugins.

Issue #2053153 by tim.plunkett, dawehner, damiankloip, aspilicious: Allow contrib modules to provide plugins on behalf of optional modules.

Issue #1903346 by Berdir, tim.plunkett, EclipseGc: Establish a new DefaultPluginManager to encapsulate best practices.

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

Issue #1987298 by tim.plunkett, quicksketch: Shorten directory structure and PSR-0 namespacing for plugins.

Issue #1930020 by dawehner, Berdir, olli: Inject a class into the Core annotated discovery implementation so that namespaces can be updated.

Issue #1836008 by dawehner, Berdir, alexpott, katbailey, effulgentsia, EclipseGc: Fixed Remove drupal_classloader() use in Drupal\Core\AnnotatedClassDiscovery.