Issue #2935255 by Jo Fitzgerald, kim.pepper, voleger, alexpott, martin107, jibran, catch: Remove all usages of drupal_get_message and drupal_set_message in core lib

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

Issue #1392754 by tss, justAChris, jhodgdon: Comply with new documentation standards for @file for namespaced class files

Issue #2234439 by jhodgdon, cosmicdreams, xjm: Add a section for the plugin system to the API doc landing page.

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

Issue #2190643 by Berdir, amateescu, Xano, yched: Serializing the container is a very very bad idea, let's prevent it?.

Needs more discussion.

This reverts commit feaee01e.

Issue #2087231 by tim.plunkett, pwolanin: Add a PluginBase in the Core namespace with t() as a helper method.