collection.dart

class IMappable {
}

class MyList<P extends IMappable> {
  final List<P> _list;
  MyList.fromJson(Map<String, dynamic> json, P func(v))
  : _list = new List<P>.from(json.values.map(func));

  P operator[] (int index) {
    return _list[index];
  }
}

class Foo implements IMappable {
  final val;
  Foo(this.val);
}


final MyList<Foo> myList = new MyList<Foo>.fromJson({'a': 'aaa', 'b': 'bbb'}, (v) => new Foo(v));


void main() {
  print(myList[0]);
  print(myList[1]);

  print(myList[0].val);
  print(myList[1].val);
}

output

Instance of 'Foo'
Instance of 'Foo'
aaa
bbb