[BUG] [Dart] anyOf of $ref enum + inline literal enum emits undeclared StatusEnum

[winrid]

The dart generator emits a property typed StatusEnum? for an anyOf between a $ref'd enum and an inline literal enum, but never declares the StatusEnum class anywhere in lib/model/, so the generated file fails to compile.

status:
  anyOf:
    - $ref: '#/components/schemas/APIStatus'
    - { type: string, enum: ['pending-verification'] }

Working on a PR.