Comment In includes/Rest/Handler.php under getOpenApiSpec()
"""
XXX: Maybe we want to be able to define a spec file in the route definition?
NOTE: the route definition may not be loaded when this is called before init()
"""
We now think this isn't a good idea. What we should do instead is provide a convenience method for loading parts of the spec from files following some naming convention. E.g. PageContentHandler.response-body-schema.yaml or something.