anchor.diff:        new ObservableList<PracticePlan>();
   }
   
  +class MissionPlan extends Observable {
  +  @observable int fleetId;
  +  @observable int missionId;
  +
  +  MissionPlan(this.fleetId, this.missionId);
  +}
  +
  +class MissionPrefs extends Observable {
  +  @observable final List<MissionPlan> missionPlans =
  +      new ObservableList<MissionPlan>();
  +}
  +
   class Preferences extends Observable {
     @observable AutomanPrefs automanPrefs = new AutomanPrefs();
     @observable FleetPrefs fleetPrefs = new FleetPrefs();
     @observable PracticePrefs practicePrefs = new PracticePrefs();
  +  @observable MissionPrefs missionPrefs = new MissionPrefs();
   
     // Convert this preferences object to JSON so that the server can directly
     // accept it as the Prefenreces object.
  
positive.code:         practicePlan["formation"]);
      prefs.practicePrefs.practicePlans.add(practicePlanObject);
    }
    model.preferences = prefs;
  }
  
negative.code:           "opponent_fleet_type": practicePlan.opponentFleetType,
            "fleet_name": practicePlan.fleetName,
            "formation": practicePlan.formation,
          }).toList(),
        },
        "mission_prefs": {
          "mission_plans": missionPrefs.missionPlans.map((missionPlan) => {
            "fleet_id": missionPlan.fleetId,
            "mission_id": missionPlan.missionId,
          }).toList(),
        },
      });
  