anchor.diff:    
     static final num pointHeight = .25;
     
  +  static final num dotRadius = 2;
  +
     static Random random;
     
     num row, col;
  +  bool isWhite;
  +  num dotCount;
     
     Tile(this.row, this.col) {
       random = new Random();
  +    this.isWhite = random.nextBool();
  +    this.dotCount = random.nextInt(3) + 1;
     }
   
     void draw(CanvasRenderingContext2D context) {
  
positive.code:     // stroke borders intersect canvas bounds
      xOffset += 1;
      yOffset += 1;
  
      var isWhite = random.nextBool();
      
      context.fillStyle = isWhite ? '#fff' : '#888';
      context.strokeStyle = '#333';
      context.beginPath();
      context.moveTo(xOffset, yOffset + pointHeight * height);
  
negative.code: import 'dart:html';
  import 'dart:math' show Random, PI;
  
  import '../lib/rect.dart';
  
  