Json Not Key-Value In Flutter

Json Not Key-Value In Flutter :

Json Not Key-Value In Flutter
class NodoCollection {
  final List<NodoPOJO> list;

  NodoCollection(this.list);

  factory NodoCollection.fromJson(List<dynamic> json) =>
      NodoCollection(json.map((e) => NodoPOJO.fromJson(e)).toList());
}

class NodoPOJO {
  final String extremo1;
  final String extremo2;
  final String linea;

  NodoPOJO(this.extremo1, this.extremo2, this.linea);

  factory NodoPOJO.fromJson(List<dynamic> json) =>
      NodoPOJO(json[0], json[1], json[2]);
}

 var jsonDecode = json.decode(
        '[["P170","P171","L-18"],["P171","L806","L-18"],["L806","L807","L-18"],["L807","L120","L-18"],["L120","L121","L-18"],["L121","L122","L-18"]]');
    NodoCollection nodoCollection = NodoCollection.fromJson(jsonDecode);

    for (var value in nodoCollection.list) {
      print(value.extremo1 + ' : ' + value.extremo2 + ' : ' + value.linea);
    }


output
P170 : P171 : L-18
P171 : L806 : L-18
L806 : L807 : L-18
L807 : L120 : L-18
L120 : L121 : L-18
L121 : L122 : L-18