anchor.diff:    @override
     bool operator [](int index) {
       RangeError.checkValidIndex(index, this);
  -    return (buffer[index >> SHIFT] & SET_MASK[index & OFFSET]) != 0;
  +    return (buffer[index >> bitShift] & bitSetMask[index & bitOffset]) != 0;
     }
   
     /// Sets the [value] of the bit with the given [index].
  
positive.code:   /// Constructs a bit list of the given [length], and initializes the
    /// value at each position with [fill].
    factory BitList.filled(int length, bool fill) {
      var buffer = new Uint32List((length + OFFSET) >> SHIFT);
      if (fill) {
        buffer.fillRange(0, buffer.length, MASK);
      }
      return new BitList._(buffer, length);
    }
  
negative.code:   /// current value.
    void flip(int index) {
      RangeError.checkValidIndex(index, this);
      buffer[index >> bitShift] ^= bitSetMask[index & bitOffset];
    }
  
    /// Counts the number of bits that are set to [expected].
    int count([bool expected = true]) {
      var tally = 0;
      for (var index = 0; index < length; index++) {
        var actual = (buffer[index >> bitShift] & bitSetMask[index & bitOffset]) != 0;
        if (actual == expected) {
          tally++;
        }
  