[BUG][DART] Polymorphism for dart-dio broken

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used? 2.0
• Have you search for related issues/PRs?
• What's the actual output vs expected output?

Description

This is a general problem with generated code for dart. Annotated classes with allOf are not identified correctly and generated twice.

For e.g. we have an Dog and Cat inherited from class Pet. The code generator would create

pet.dart
dog.dart
dog_all_off.dart
cat.dart
cat_all_off.dart

To support polymorphism in general, it would be needed to match this built_value example

openapi-generator version

Using latest snapshot 4.3.1.

OpenAPI declaration file content or url

https://pastebin.com/f4ied29Q

Command line used for generation

java -jar openapi-generator-cli-4.3.1-20200423.060155-118.jar generate -i swagger.json -c config.json -l dart-dio -o .

Steps to reproduce

Generate the code

Start by reproducing the issue with the listed openapi-generator-cli command, swagger.json, and config.json, using the declaration at the linked Pastebin URL. Compare the generated pet.dart, dog.dart, dog_all_off.dart, cat.dart, and cat_all_off.dart files with the intended Pet, Dog, and Cat hierarchy. Done means allOf-derived classes are identified correctly without duplicate generated classes and polymorphism works as described.