ConstructorDefinitionBuilder.augment cannot augment factory constructor

[blaugold]

When I try to use ConstructorDefinitionBuilder.augment with a previously declared factory constructor, I get the following error for the augmentation: All final variables must be initialized, but 'bar', 'buzz', and 2 others aren't. Try adding initializers for the fields.

Here is the generated code:

augment class A {
  external factory A.readJson(prefix0.JsonReader reader);

  augment A.readJson(prefix0.JsonReader reader, ) {
    ...
  }
}