[BUG] [dart-dio-next] `readOnly: true` is not considered for required fields

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

BuiltValues are validating for null if the field is added to required fields list. But if the field is readOnly: true, built value should not throw error if we are using the data for write operation. Any suggestions on how to achieve this?

openapi-generator version

openapi-generator-cli 5.2.1
commit : 8403e59

OpenAPI declaration file content or url

If you post the code inline, please wrap it with

components:
  schemas:
    RefreshCredentials:
      type: object
      properties:
        access:
          type: string
          readOnly: true
        expires_in:
          type: number
          format: float
          readOnly: true
        refresh:
          type: string
          writeOnly: true
      required:
      - access
      - expires_in
      - refresh

Generation Details

generator: dart-dio-next

Steps to reproduce

run generator, then run build runner.

refreh_credentials.dart looks fine

  @BuiltValueField(wireName: r'access')
  String get access;

  @BuiltValueField(wireName: r'expires_in')
  double get expiresIn;

  @BuiltValueField(wireName: r'refresh')
  String get refresh;

but generated refresh_credentials.g.dart doesn't:

...
  @override
  _$RefreshCredentials build() {
    final _$result = _$v ??
        new _$RefreshCredentials._(
            access: BuiltValueNullFieldError.checkNotNull(
                access, 'RefreshCredentials', 'access'),                // <----- THIS LINE
            expiresIn: BuiltValueNullFieldError.checkNotNull(
                expiresIn, 'RefreshCredentials', 'expiresIn'),                // <----- THIS LINE
            refresh: BuiltValueNullFieldError.checkNotNull(
                refresh, 'RefreshCredentials', 'refresh'));
    replace(_$result);
    return _$result;
  }

Lines pointed above are to ensure access/expiresIn are not null while they are supposed to be when building the value for write.

Reproduce the issue with the provided OpenAPI declaration using the dart-dio-next generator, then run build runner and compare refresh_credentials.dart with refresh_credentials.g.dart. Trace how required and readOnly fields become BuiltValueNullFieldError checks. Done means readOnly required fields no longer fail validation when building the value for a write operation.