[Dart] Enum support

[cputoaster]

Description

Enum support for Dart is currently not working. There is a non-working partial implementation available that still uses the old dartson way.

Swagger-codegen version

git master at a5e26a4

Swagger declaration file content or url

"ACTION": {
   "enum": [
    "NONE",
    "LOGOUT",
    "UPGRADE",
    "RESET_CLIENT_DB",
    "RELOAD_PAGE",
    "REGISTER"
   ]
  },

Command line used for generation

java -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate -i openapi.json -l dart -o api -DbrowserClient=false

Steps to reproduce

• create openapi.json with an enum
• use swagger-codgen to create dart api code
• results in something non-working like this:

@Entity()
class ACTION {
  /// The underlying value of this enum member.
  final  value;

  const ACTION._internal(this.value);

  static const ACTION nONE_ = const ACTION._internal("NONE");
  ...
  static const ACTION rEGISTER_ = const ACTION._internal("REGISTER");
}

class ACTIONTypeTransformer extends TypeTransformer<ACTION> {

  @override
  dynamic encode(ACTION data) {
    return data.value;
  }

  @override
  ACTION decode(dynamic data) {
    switch (data) {
      case &quot;NONE&quot;: return ACTION.nONE_;
      ...
      case &quot;REGISTER&quot;: return ACTION.rEGISTER_;
      default: throw('Unknown enum value to decode: $data');
    }
  }
}

Related issues/PRs

first mentioned in:
#7418

Suggest a fix/enhancement

see
#7418 (comment)

[cputoaster]

Duplicate of #5454