anchor.diff:    expect(direction.y, equals(y));
   }
   
  -void testAddToMainDirections(DirectionSystem directionSystem, List directions, List<Map> expectedDirections, bool doNullMainDirection) {
  +void testAddToMainDirection(DirectionSystem directionSystem, List directions, List<Map> expectedDirections, bool doNullMainDirection) {
     for (int i = 0; i < directions.length; i++) {
       directionSystem.addToMainDirection(directions[i]);
       testDirection(directionSystem.mainDirection, expectedDirections[i]["x"], expectedDirections[i]["y"]);
  
positive.code:       directionSystem = null;
      });
      test("Testing main directions", () {
        testAddToMainDirections(directionSystem, Direction.MAIN_DIRECTIONS, [{
            "x": 0,
            "y": -1
          }, {
  
negative.code:         }], true);
      });
      test("Testing diagonal directions", () {
        testAddToMainDirection(directionSystem, Direction.DIAGONAL_DIRECTIONS, [{
            "x": -expectedDiagonalValue,
            "y": -expectedDiagonalValue
          }, {
  