[BUG] [Dart] Object.mapFromJson method not found error in Map<String, Object> attribute deserialization

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Generation produces a compiler error because Map<String, Object> attributes are deserialized with Object.mapFromJson method that doesn't exist:

class Post {
  
  Map<String, Object> customData;
  
  static Post fromJson(Map<String, dynamic> json) => json == null
    ? null
    : Post(
        title: json[r'title'],
        customData: json[r'customData'] == null
          ? null
          : Object.mapFromJson(json[r'customData']),
    );
  
}

openapi-generator version

Version 5.1.0

OpenAPI declaration file content or url

swagger: "2.0"
info:
  version: 1.0.0
  title: Swagger Petstore
  description: >
    A sample API that uses a petstore as an example
    to demonstrate features in the swagger-2.0 specification
consumes:
  - application/json
produces:
  - application/json
paths:
  /post:
    get:
      responses:
        "200":
          description: pet response
          schema:
            type: array
            items:
              $ref: "#/definitions/Post"
definitions:
  Post:
    type: object
    properties:
      title:
        type: string
      customData:
        type: object
        additionalProperties:
          type: object

Generation Details

java -jar openapi-generator-cli-5.1.0.jar \
    generate -g dart \
    -i post.yml \
    -o post

Steps to reproduce

Related issues/PRs

#7850
https://github.com/OpenAPITools/openapi-generator/issues/8179

Suggest a fix

I found that adding Object as a primitive type in modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractDartCodegen.java:114 produces the correct behaviour:

languageSpecificPrimitives = Sets.newHashSet(
                "Object", // added as a primitive type
                "String",
                "bool",
                "int",
                "num",
                "double",
                "dynamic"
        );

And generated model:

class Post {
  
  Map<String, Object> customData;
  
  static Post fromJson(Map<String, dynamic> json) => json == null
      ? null
      : Post(
          title: json[r'title'],
          customData: json[r'customData'] == null
              ? null
              : (json[r'customData'] as Map).cast<String, Object>(),
        );
  
}

Start in modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractDartCodegen.java around line 114, then reproduce with the provided OpenAPI YAML and generation command. Done means generated Dart deserialization for Map<String, Object> uses the shown map cast rather than calling the nonexistent Object.mapFromJson method.