Issue #2332389 by tim.plunkett: Finish adding methods to FormStateInterface.

core/includes/install.core.inc

core/lib/Drupal/Core/Block/BlockBase.php

core/lib/Drupal/Core/Entity/EntityFormBuilder.php

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

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

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

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

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

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

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

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

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

...ests/src/Unit/Plugin/AggregatorPluginSettingsBaseTest.php

core/modules/block/src/BlockForm.php

core/modules/config/src/Form/ConfigSingleExportForm.php

core/modules/image/src/Form/ImageEffectFormBase.php

core/modules/menu_ui/src/MenuForm.php

core/modules/quickedit/src/QuickEditController.php

core/modules/simpletest/src/Form/SimpletestResultsForm.php

.../modules/system/src/Tests/Element/PathElementFormTest.php