samples/poppopwin: upgrade to first-class sample

samples/pop_pop_win/lib/src/poppopwin/field.dart

 part of ppw;
 
 class Field extends Array2d<bool> {
   final int bombCount;
   final Array2d<int> _adjacents;
 
   factory Field([bombCount = 40, cols = 16, rows = 16, int seed = null]) {
     final squares = new List<bool>.filled(rows * cols, false);
     assert(bombCount < squares.length);
     assert(bombCount > 0);
@@ skipping 65 matching lines @@
           val++;
         }
       }
       _adjacents.set(x, y, val);
     }
     return val;
   }
 
   String toString() => 'w${width}h${height}m${bombCount}';
 }