Optional and optional named query parameters are not handled properly

[Aidanvii7]

Description
This is arguably a feature request, however it can be considered a bug as well as the optional and optional named parameters are accepted.

To Reproduce
Implement something like:

import 'package:dio/dio.dart';
import 'package:retrofit/http.dart';
import 'package:meta/meta.dart';

part 'my_client.g.dart';

@RestApi()
abstract class MyClient {

  factory MyClient(Dio dio) = _MyClient;

  @GET("content/api")
  Future<Result> namedExample(@Query("apikey") String apiKey,
      @Query("scope") String scope,
      @Query("type") String type, {
        @required @Query("from") int from
      });

  @GET("content/api")
  Future<Result> optionalNamedExample(@Query("apikey") String apiKey,
      @Query("scope") String scope,
      @Query("type") String type, {
        @Query("from") int from
      });

  @GET("content/api")
  Future<Result> optionalExample(@Query("apikey") String apiKey,
      @Query("scope") String scope,
      @Query("type") String type, [
        @Query("from") int from
      ]);
}

Expected behavior
In the case of namedExample, I would expect a null assertion to occur for the from parameter as of the existence of the @required annotation, however this is not the case.

In the case of optionalNamedExample, I would expect no null assertion for the from parameter as @required is not present. This is correct, however I would expect it to only be added to the queryParameters if from is not null (I think Square's Retrofit handles this). This could be done simply with Dart 2.3's control flow collections by doing:

final queryParameters = <String, dynamic>{
      'apikey': apiKey,
      'scope': scope,
      'type': type,
      if(from != null) 'from': from
    };

In the case of optionalExample, I would expect the same sort of behaviour as above.

[Aidanvii7]

p.s. great job with the library!

[devkabiir]

@required is deprecated, Work is in progress for the new required syntax see: dart-lang/language#15.

Temporarily I can enable the @required But It will go away in the future.

[marinat]

is this realized? optional query parameter still adds even it is null