17 files declare their use of FormBase

ClearCacheForm.php in core/modules/system/src/Form/ClearCacheForm.php
CopyFieldValueTestForm.php in core/modules/system/tests/modules/system_test/src/Form/CopyFieldValueTestForm.php
DevelopmentSettingsForm.php in core/modules/system/src/Form/DevelopmentSettingsForm.php
EmbeddedForm.php in core/modules/field_layout/tests/modules/field_layout_test/src/Form/EmbeddedForm.php
EntityForm.php in core/lib/Drupal/Core/Entity/EntityForm.php
EntityResolverManagerTest.php in core/tests/Drupal/Tests/Core/Entity/EntityResolverManagerTest.php
FieldStorageReuseForm.php in core/modules/field_ui/src/Form/FieldStorageReuseForm.php
FileTransferAuthorizeForm.php in core/lib/Drupal/Core/FileTransfer/Form/FileTransferAuthorizeForm.php
FormSubmitterTest.php in core/tests/Drupal/Tests/Core/Form/FormSubmitterTest.php
FormTestDetailsContainsRequiredTextfieldForm.php in core/modules/system/tests/modules/form_test/src/Form/FormTestDetailsContainsRequiredTextfieldForm.php
LayoutForm.php in core/modules/navigation/src/Form/LayoutForm.php
MessageForm.php in core/modules/migrate/src/Form/MessageForm.php
SelectLanguageForm.php in core/lib/Drupal/Core/Installer/Form/SelectLanguageForm.php
SelectProfileForm.php in core/lib/Drupal/Core/Installer/Form/SelectProfileForm.php
SiteSettingsForm.php in core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php
TestForm.php in core/modules/system/tests/modules/dialog_renderer_test/src/Form/TestForm.php
UserLanguageTestForm.php in core/modules/user/tests/modules/user_language_test/src/Form/UserLanguageTestForm.php

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