Issue #2105661 by dawehner, tim.plunkett, chx: Add support for $form_state()['redirect_route()'].

core/lib/Drupal/Core/Form/FormBuilder.php

...les/action/lib/Drupal/action/ActionFormControllerBase.php

...s/action/lib/Drupal/action/Form/ActionAdminManageForm.php

...odules/action/lib/Drupal/action/Form/ActionDeleteForm.php

...iews_ui/lib/Drupal/views_ui/Form/Ajax/ReorderDisplays.php

core/tests/Drupal/Tests/Core/Form/FormBuilderTest.php