Short answer: Restart apache or disable the APC classloader in your local.settings.php (see settings.php).
While working on the Give module, which follows the same pattern for donation forms as Drupal 8 core's Contact module uses for contact forms, an access control handler was not being found even though i quadruple checked that my handler annotation described the handler correctly for the name and location of its file.
The ConfigEntityType annotation for the access handler, with most other annotations removed:
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