Issue #3364506 by Wim Leers, effulgentsia, catch, lauriii, borisson_, smustgrave, tim.plunkett, Dave Reid: Add optional validation constraint support to ConfigFormBase

Issue #2935256 by Jo Fitzgerald, voleger, kim.pepper, ibustos, Yogesh Pawar, catch, jibran: Remove all usages of drupal_get_message and drupal_set_message in modules

Issue #2873696 by nvexler, wengerk, gido, Jo Fitzgerald, xjm: Add @internal to core classes, methods, properties to Form Classes

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 #2392319 by alexpott, effulgentsia: Config objects (but not config entities) should by default be immutable

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

Issue #1998140 by dawehner, ParisLiakos, tim.plunkett: Remove backward compatible ControllerInterface.

Issue #1998166 by dawehner: Use the controller resolver to inject parameters into HtmlFormController.

Issue #1913084 by tim.plunkett: Introduce a Form interface for building, validating, and submitting forms.

Issue #1913084 by tim.plunkett: Introduce a Form interface for building, validating, and submitting forms.