samples/poppopwin: upgrade to first-class sample

samples/pop-pop-win/test/ppw/test_field.dart

 part of ppw_test;
 
 class TestField {
   // This grid
   // XXXXX2
   // X7X8X3
   // X5XXX2
   // X32321
   // 110000
 
@@ skipping 56 matching lines @@
 
     for(int x = 0; x < f.width; x++) {
       for(int y = 0; y < f.height; y++) {
         final i = x + y * f.width;
         final adj = f.getAdjacentCount(x, y);
         expect(adj, equals(sample[i]));
       }
     }
   }
 }