Note: this list is generated by looking for the string for this theme hook, so it may include some references that are not actually using this theme hook.

BackdropWebTestCase::handleForm in core/modules/simpletest/backdrop_web_test_case.php
Handle form input related to backdropPost(). Ensure that the specified fields exist and attempt to create POST data in the correct manner for the particular field type.
backdrop_common_theme in core/includes/common.inc
Provides theme registration for themes across .inc files.
form_test_menu in core/modules/simpletest/tests/form_test.module
Implements hook_menu().
form_test_menu in core/modules/simpletest/tests/form_test.module
Implements hook_menu().
system_element_info in core/modules/system/system.module
Implements hook_element_info().
_form_test_disabled_elements in core/modules/simpletest/tests/form_test.module
Build a form to test disabled elements.