[Dart] Fix incorrect generation of additionalProperties:true (#12165)

[0xNF]

fixes #12165

This PR includes a patch to the modules/openapi-generator/src/main/resources/dart2/serialization/native/native_class.mustache file which previously wasn't handling the deserialiation of Map<String, Object> types correctly.

Previously, the deserialization of such fields was being generated as mapValueOfType<Map<String, Object>>(json, key), but due to Dart using different internal map representations, this would always fail at runtime, meaning that such objects were not able to be generated by this project.

This change adjusts the mustache file to detect when a field is a complex map and uses mapCastOfType<String, Object>(json[key]) instead, which does not crash at runtime.

The correctness of the change can be confirmed using the following test files:

spec.yaml:

openapi: 3.0.3
info:
  version: "1.1"
  title: Dart AdditionaProperties True
servers:
  - url: 'localhost'
    variables:
      host:
        default: localhost
components:
  schemas:
    ItemsWithMapStringAnyWithPropsTrue:
      type: object
      description: "map-string-any defined with `additionalProperties: true`"
      properties:
        objectMap:
          type: object
          additionalProperties: true
    ItemWithMapStringAnyWithPropsObject:
      type: object
      description: "map-string-any defined with `additionalProperties: object`"
      properties:
        objectMap:
          type: object
          additionalProperties:
            type: object
paths:
  /items:
    get:
      operationId: GetItemWithMapStringObjects
      responses:
        "200":
          description: A list of ItemWithMapStringObject
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ItemWithMapStringAnyWithPropsObject'

Dart test file

//
// AUTO-GENERATED FILE, DO NOT MODIFY!
//
// @dart=2.12

// ignore_for_file: unused_element, unused_import
// ignore_for_file: always_put_required_named_parameters_first
// ignore_for_file: constant_identifier_names
// ignore_for_file: lines_longer_than_80_chars

import 'dart:convert';

import 'package:openapi/api.dart';
import 'package:test/test.dart';

// tests for ItemWithMapStringAnyWithPropsObject
void main() {
  // final instance = ItemWithMapStringAnyWithPropsObject();

  final String s = '{"objectMap": {"item_int": 1, "item_bool": true, "item_string": "str", "item_double": 4.3, "item_map": {"subProps": 1}, "item_array": [-1]}}';

  group('test ItemWithMapStringAnyWithPropsObject', () {
    // Map<String, Object> objectMap (default value: const {})
    test('to test the property `objectMap`', () async {
      final m = JsonDecoder().convert(s);
      final i = ItemsWithMapStringAnyWithPropsTrue.fromJson(m)!;
      assert(i.objectMap["item_int"] == 1);
      assert(i.objectMap["item_bool"] == true);
      assert(i.objectMap["item_string"] == "str");
      assert(i.objectMap["item_double"] == 4.3);
      assert((i.objectMap["item_map"] as Map)["subProps"] == 1);
      assert((i.objectMap["item_array"] as List)[0] == -1);
    });

    test('to test the property `objectMap`', () async {
      final m = JsonDecoder().convert(s);
      final i = ItemWithMapStringAnyWithPropsObject.fromJson(m)!;
      assert(i.objectMap["item_int"] == 1);
      assert(i.objectMap["item_bool"] == true);
      assert(i.objectMap["item_string"] == "str");
      assert(i.objectMap["item_double"] == 4.3);
      assert((i.objectMap["item_map"] as Map)["subProps"] == 1);
      assert((i.objectMap["item_array"] as List)[0] == -1);
    });
  });
}

@jaumard @josh-burton @amondnet @sbu-WBT @kuhnroyal @agilob @ahmednfwela

[0xNF]

The logs for the failed test show that AWS's US-East-1 was failing to access some URLs -- otherwise this PR looks like it passes all the requisite automatic checks:

 Failed to fetch http://us-east-1.ec2.archive.ubuntu.com/ubuntu/pool/main/g/glibc/libc6-dbg_2.31-0ubuntu9.9_amd64.deb  503  Service Unavailable [IP: 52.91.65.63 80]