anchor.diff:  class RandomStrategy extends Strategy {
     static final Random _random = new Random();
   
  -  const RandomStrategy(): super("random");
  +  const RandomStrategy() : super("random");
   
     String nextDirection(Board board) {
       return Board.DIRECTIONS[_random.nextInt(Board.DIRECTIONS.length)];
  
positive.code:   final int searchDepth;
    final bool sumScores;
  
    const EvaluateStrategy(String name, this.searchDepth, [this.sumScores = false]): super(name);
  
    num evaluate(Board board, List<String> moves) {
      return board.score;
  
negative.code: }
  
  class EdgeStrategy extends EvaluateStrategy {
    const EdgeStrategy(int searchDepth) : super("edge $searchDepth", searchDepth, true);
  
    num _cellScore(Cell cell) {
      if (cell.empty) {
  