main.dart

import 'package:http/http.dart' as http;
import 'dart:convert';

import 'package:json/json.dart';

main() async {
  const pubUrl = "https://pub.dartlang.org/api/packages/protobuf";
  var response = await http.get(Uri.parse(pubUrl));
  if (response.statusCode == 200) {
    PkgInfo info = PkgInfo.fromJson(json.decode(response.body));
    print('Package ${info.name}, latest is v${info.latest.pubspec.version}');
    print(info.latest.pubspec.description);
  } else {
    throw Exception('Failed to load package info');
  }
}

@JsonCodable()
class PkgInfo({required final String name, required final PkgVersion latest});

@JsonCodable()
class PkgVersion({required final String version, required final PkgPubspec pubspec});

@JsonCodable()
class PkgPubspec({required final String version, required final String description});

We could also use const if nothing else prevents constant construction:

@JsonCodable()
class const PkgInfo({required String name, required PkgVersion latest});

@JsonCodable()
class const PkgVersion({required String version, required PkgPubspec pubspec});

@JsonCodable()
class const PkgPubspec({required String version, required String description});

Oh, and if we make it possible to omit required in concrete function signatures (not function types, not abstract method declarations) in the case where the formal parameter type is non-nullable then we get this:

@JsonCodable()
class const PkgInfo({String name, PkgVersion latest});

@JsonCodable()
class const PkgVersion({String version, PkgPubspec pubspec});

@JsonCodable()
class const PkgPubspec({String version, String description});