samples/poppopwin: upgrade to first-class sample

samples/third_party/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);
-
-    final rnd = new math.Random(seed);
-
-    // This is the most simple code, but it'll get slow as
-    // bombCount approaches the square count.
-    // But more efficient if bombCount << square count
-    // which is expected.
-    for(int i = 0; i < bombCount; i++) {
-      int index;
-      do {
-        index = rnd.nextInt(squares.length);
-      } while(squares[index]);
-      squares[index] = true;
-    }
-
-    return new Field._internal(bombCount, cols,
-        new ReadOnlyCollection<bool>(squares));
-  }
-
-  factory Field.fromSquares(int cols, int rows, List<bool> squares) {
-    assert(cols > 0);
-    assert(rows > 0);
-    assert(squares.length == cols * rows);
-
-    int count = 0;
-    for(final m in squares) {
-      if(m) {
-        count++;
-      }
-    }
-    assert(count > 0);
-    assert(count < squares.length);
-
-    return new Field._internal(count, cols,
-        new ReadOnlyCollection<bool>(squares));
-  }
-
-  Field._internal(this.bombCount, int cols, ReadOnlyCollection<bool> source) :
-    this._adjacents = new Array2d<int>(cols, source.length ~/ cols),
-    super.wrap(cols, source.toList()) {
-    assert(width > 0);
-    assert(height > 0);
-    assert(bombCount > 0);
-    assert(bombCount < length);
-
-    int count = 0;
-    for(final m in this) {
-      if(m) {
-        count++;
-      }
-    }
-    assert(count == bombCount);
-  }
-
-  int getAdjacentCount(int x, int y) {
-    if(get(x,y)) {
-      return null;
-    }
-
-    int val = _adjacents.get(x, y);
-
-    if(val == null) {
-      val = 0;
-      for(final i in getAdjacentIndices(x,y)) {
-        if(this[i]) {
-          val++;
-        }
-      }
-      _adjacents.set(x, y, val);
-    }
-    return val;
-  }
-
-  String toString() => 'w${width}h${height}m${bombCount}';
-}