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

Issue #2508884 by EclipseGc, tim.plunkett, dsnopek, japerry, sravya_12, swathin, Wim Leers, Fabianx, dawehner, yched: Make contexts immutable

Issue #2513244 by Berdir, Wim Leers, EclipseGc, Fabianx: ContextHandler incorrectly checks required/optional contexts of plugins

Issue #2292889 by Berdir, tim.plunkett: Fixed ContextHandler does not expect ContextDefinitionInterface.

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