[dart-dio] correctly handle Map<String, Object>, List<Object> using JsonObject

Created by: amondnet

https://github.com/google/built_value.dart/issues/134

dart-dio generator does not handle Map<String, dynamic> properly. This PR fixes this issue using JsonObject.

• Map<String, Object> should be BuiltMap<String,JsonObject>
• List<Object> should be BuiltList<JsonObject>

  Test:
    properties:
      objectMap:
        type: object
        additionalProperties:
          type: object
      objectList:
        type: array
        items:
          type: object

  @nullable
  @BuiltValueField(wireName: 'objectMap')
  BuiltMap<String, JsonObject> get objectMap;

  @nullable
  @BuiltValueField(wireName: 'objectList')
  BuiltList<JsonObject> get objectList;

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

CC: @ircecho @swipesight @jaumard @nickmeinhold @athornz