First things on Dart

main.dart

import 'dart:convert';
import 'package:http/http.dart';
import 'package:flutter/foundation.dart';

const String serviceUrl = 'https://jsonplaceholder.typicode.com';

class HttpCallService extends HttpCallService {
  String path;

  HttpCallService(this.path);

  Future<List<dynamic>> get(String endpoint) async {
    try {  
      Response res = await get(serviceUrl + this.path + endpoint);
      return List<dynamic> jsonDecode(res.body);
    }
    catch(e) { 
      throw new FormatException(e); 
    } 
  }    
}

class PostDto {
  final int id;
  final int userId;
  final String body;
  final String title;

  Post({
    @required this.id,
    @required this.userId,
    @required this.body,
    @required this.title,
  });

  factory Post.fromJson(Map<String, dynamic> json) {
    return Post(
      id: json['id'] as int,
      userId: json['userId'] as int,
      body: json['body'] as String,
      title: json['title'] as String,
    );
  }
}

class PostService extends HttpCallService {
  String path;

  super.HttpCallService(this.path);

  Future<List<PostDto>> getPosts(String endpoint) async {
    try {  
      List<dynamic> resJson = await this.HttpCallService.get(endpoint);
      List<Post> posts = resJson.map((dynamic item) => PostDto.fromJson(item)).toList();
      return posts
    }
    catch(e) { 
      throw new FormatException(e); 
    } 
  }    
}

void main() {
  postService = PostService('/posts');
  posts =postService.getPosts('/');
  posts.forEach((p) => print(p.title));
}