Impossible to create Dart client with Github spec

Description

Hi. I am tried to use OpenApitools and https://editor.swagger.io/ online generators for build dart github api client, but no one of them cant build working code.

openapi-generator version

5.0.0-beta2

OpenAPI declaration file content or url

https://github.com/github/rest-api-description/tree/main/descriptions/api.github.com

Command line used for generation

I tried using online generator on https://editor.swagger.io/,
just put URL on github json spec file and select DART in dropdown menu.

After that i also try to use openapi tools generator and download jar and used command from
https://github.com/OpenAPITools/openapi-generator/wiki/API-client-generator-HOWTO

java -jar .\openapi-generator-cli-5.0.0-beta2.jar generate -i .\openapi.json -g dart -o .\dart-client

Where openapi.json file i getting from https://github.com/github/rest-api-description/tree/main/descriptions/api.github.com

Steps to reproduce

Just try to build client from github swagger.json, and open code editor, after analyze is finished dart compiler will show all issues with generated code.

Reproduce the generation using the linked GitHub OpenAPI declaration, OpenAPI Generator 5.0.0-beta2, and the documented generate -i ... -g dart command. Inspect the Dart compiler errors after generation; the issue is done when the generated GitHub client builds successfully.