[dart-dio] Fix compile error in enum properties with "default" values (#20495)

[chizhanov]

This PR solves the problem described in #20495.

I made this fix locally and on my openapi the issue was fixed and no new issues were found.

@jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

[kuhnroyal]

Do we have this in one of the samples? Probably not...

[chizhanov]

Do we have this in one of the samples? Probably not...

I didn't see it, but we can try to add it:

OpenAPI spec:

{
  "openapi": "3.1.0",
  "info": {
    "title": "Simple",
    "version": "0.0.1"
  },
  "paths": {},
  "components": {
    "schemas": {
      "TestEnum": {
        "type": "string",
        "enum": [
          "",
          "1",
          "2"
        ],
        "title": "TestEnum"
      },
      "TestItem": {
        "properties": {
          "test": {
            "type": "integer",
            "title": "test"
          },
          "testEmum": {
            "$ref": "#/components/schemas/TestEnum",
            "default": ""
          }
        },
        "type": "object",
        "required": [
          "test"
        ],
        "title": "TestItem"
      }
    }
  },
  "tags": []
}

And problem will be here:

@BuiltValueHook(initializeBuilder: true)
  static void _defaults(TestItemBuilder b) => b
      ..testEmum = const ._(TestEnum.empty);

[barryguvenkaya]

@ahmednfwela would be great to get this reviewed ASAP please because it affects us too.

[ahmednfwela]

@wing328 can this case be added to any of the samples we use? unfortunately they are all shared with other generators

[wing328]

@chizhanov did you test it locally to confirm it works for you?

i can add a test or 2 later using a dedicated for dart client tests.

[wing328]

i just merged #21516 to the master to have dart dedicated test spec (fake petstore)

can you please merge the latest master into your branch and add a test schema with enum properties (with default values) and regenerate the samples (step 3 in pr checklist) when you've time?

dart test spec: modules/openapi-generator/src/test/resources/3_0/dart/petstore-with-fake-endpoints-models-for-testing.yaml