anchor.diff:  import 'dart:collection';
   import 'dart:typed_data';
   
  -// TODO(renggli): use UnmodifiableListMixin<bool> when available
  +// TODO(renggli): use FixedLengthListMixin<bool> when available
   class BitSet extends ListBase<bool> {
   
     static final _SET_MASK = new Uint32List.fromList([1, 2, 4, 8,
  
positive.code:     if (0 <= index && index < length) {
        return (_buffer[index >> 5] & _SET_MASK[index & 31]) != 0;
      } else {
        throw new RangeError.value(index);
      }
    }
  
  
negative.code:         _buffer[index >> 5] &= _CLR_MASK[index & 31];
        }
      } else {
        throw new RangeError.range(index, 0, length - 1);
      }
    }
  
  