Push objects into array in Dart

I will suggest to use

returnMovies.addAll({your object here})

When you do this json.decode(response.body) you are getting a List of Map you should use List<dynamic> movieListData and get the items like this:

    movieListData = json.decode(response.body);
    returnMovies = movieListData.map((dynamic movieData) {
          String id = movieData['_id'];
          String host_group_name = movieData['host_group_name'];
          String duration = movieData['duration'];
          return new Movie(id,title, duration);
        }).toList();

Sounds like you are looking for addAll

returnMovies.addAll(json.decode(returnUpcoming.body))