anchor.diff:    @override
     bool operator [] (int index) {
       if (0 <= index && index < length) {
  -      return (_buffer[index >> 5] & _SET_MASK[index & 31]) != 0;
  +      return (_buffer[index >> _SHIFT] & _SET_MASK[index & _OFFSET]) != 0;
       } else {
         throw new RangeError.range(index, 0, length);
       }
  
positive.code:   int count([bool expected = true]) {
      int tally = 0;
      for (var index = 0; index < _length; index++) {
        var actual = (_buffer[index >> 5] & _SET_MASK[index & 31]) != 0;
        if (actual == expected) {
          tally++;
        }
  
negative.code:  */
  class BitList extends ListBase<bool> with FixedLengthListMixin<bool>  {
  
    // constants specific to mapping bits into a [Uint32List]
    static final int _SHIFT = 5;
    static final int _OFFSET = 31;
    static final _SET_MASK = new Uint32List.fromList([1, 2, 4, 8,
        16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384,
        32768, 65536, 131072, 262144, 524288, 1048576, 2097152,
  