25 files declare their use of FormInterface

ActionAdminManageForm.php in drupal/core/modules/action/lib/Drupal/action/Form/ActionAdminManageForm.php: Contains \Drupal\action\Form\ActionAdminManageForm.
BanAdmin.php in drupal/core/modules/ban/lib/Drupal/ban/Form/BanAdmin.php: Contains \Drupal\ban\Form\BanAdmin.
BlockListController.php in drupal/core/modules/block/lib/Drupal/block/BlockListController.php: Contains \Drupal\block\BlockListController.
ConditionInterface.php in drupal/core/lib/Drupal/Core/Condition/ConditionInterface.php: Contains \Drupal\Core\Condition\ConditionInterface.
ConfigExportForm.php in drupal/core/modules/config/lib/Drupal/config/Form/ConfigExportForm.php
ConfigImportForm.php in drupal/core/modules/config/lib/Drupal/config/Form/ConfigImportForm.php
DateFormatFormBase.php in drupal/core/modules/system/lib/Drupal/system/Form/DateFormatFormBase.php: Contains \Drupal\system\Form\DateFormatFormBase.
FieldEditForm.php in drupal/core/modules/field_ui/lib/Drupal/field_ui/Form/FieldEditForm.php: Contains \Drupal\field_ui\Form\FieldEditForm.
FieldInstanceFormBase.php in drupal/core/modules/field_ui/lib/Drupal/field_ui/Form/FieldInstanceFormBase.php: Contains \Drupal\field_ui\Form\FieldInstanceFormBase.
FileTestForm.php in drupal/core/modules/file/tests/file_test/lib/Drupal/file_test/Form/FileTestForm.php: Contains \Drupal\file_test\Form\FileTestForm.
form.inc in drupal/core/includes/form.inc: Functions for form and batch generation and processing.
FormController.php in drupal/core/modules/system/tests/modules/condition_test/lib/Drupal/condition_test/FormController.php: Contains \Drupal\condition_test\FormController.
FormTestArgumentsObject.php in drupal/core/modules/system/tests/modules/form_test/lib/Drupal/form_test/FormTestArgumentsObject.php: Contains \Drupal\form_test\FormTestArgumentsObject.
FormTestControllerObject.php in drupal/core/modules/system/tests/modules/form_test/lib/Drupal/form_test/FormTestControllerObject.php: Contains \Drupal\form_test\FormTestControllerObject.
FormTestObject.php in drupal/core/modules/system/tests/modules/form_test/lib/Drupal/form_test/FormTestObject.php: Contains \Drupal\form_test\FormTestObject.
FormTestServiceObject.php in drupal/core/modules/system/tests/modules/form_test/lib/Drupal/form_test/FormTestServiceObject.php: Contains \Drupal\form_test\FormTestServiceObject.
OpmlFeedAdd.php in drupal/core/modules/aggregator/lib/Drupal/aggregator/Form/OpmlFeedAdd.php: Contains \Drupal\aggregator\Form\OpmlFeedAdd.
OverviewBase.php in drupal/core/modules/field_ui/lib/Drupal/field_ui/OverviewBase.php: Contains \Drupal\field_ui\OverviewBase.
RoleListController.php in drupal/core/modules/user/lib/Drupal/user/RoleListController.php: Contains \Drupal\user\RoleListController.
SimpletestResultsForm.php in drupal/core/modules/simpletest/lib/Drupal/simpletest/Form/SimpletestResultsForm.php: Contains \Drupal\simpletest\Form\SimpletestResultsForm.
SimpletestTestForm.php in drupal/core/modules/simpletest/lib/Drupal/simpletest/Form/SimpletestTestForm.php: Contains \Drupal\simpletest\Form\SimpletestTestForm.
SystemConfigFormBase.php in drupal/core/modules/system/lib/Drupal/system/SystemConfigFormBase.php: Contains \Drupal\system\SystemConfigFormBase.
ViewsFormInterface.php in drupal/core/modules/views_ui/lib/Drupal/views_ui/Form/Ajax/ViewsFormInterface.php: Contains \Drupal\views_ui\Form\Ajax\ViewsFormInterface.
ViewsTestDataElementForm.php in drupal/core/modules/views/tests/views_test_data/lib/Drupal/views_test_data/Form/ViewsTestDataElementForm.php: Contains \Drupal\views_test_data\Form\ViewsTestDataElementForm.
VocabularyListController.php in drupal/core/modules/taxonomy/lib/Drupal/taxonomy/VocabularyListController.php: Contains \Drupal\taxonomy\VocabularyListController.