anchor.diff:      if (_bitsPerElement == 8) {
         return new Uint32List(capacity);
       }
  +    assert(false);
       return null;
     }
     void _shrinkSize() {
  
positive.code:   int _bitsPerElement;
    List<int> _elements;
    int _usedElements;
  
    VariableLengthBitSet([int bitCount = 0, int bitsPerElement]) : super.base() {
      if (bitCount == null) {
  
negative.code:     }
      return count;
    }
    int _bitIndexToElementIndex(int bitIndex) {
      if (_bitsPerElement == 64) {
        return bitIndex >> 6;
      } else if (_bitsPerElement == 32) {
        return bitIndex >> 5;
      } else if (_bitsPerElement == 16) {
        return bitIndex >> 4;
      } else if (_bitsPerElement == 8) {
        return bitIndex >> 3;
      }
      assert(false);
      return 0;
    }
    int _bitIndexToElementBitIndex(int bitIndex) {
      return bitIndex & _elementBitIndexMask;
    }
    void setBit(int index) {
      if (index < 0) {
        throw new RangeError("Index ($index) must be greater than or equal to 0.");
      }
      int elementIndex = _bitIndexToElementIndex(index);
      if (elementIndex >= _elements.length) {
        _ensureCapacity(elementIndex + 1);
      }
      _elements[elementIndex] |= 1 << _bitIndexToElementBitIndex(index);
      _usedElements = max(_usedElements, elementIndex + 1);
    }
    void clearBit(int index) {
      if (index < 0) {
        throw new RangeError("Index ($index) must be greater than or equal to 0.");
      }
      int elementIndex = _bitIndexToElementIndex(index);
      if (elementIndex >= _elements.length) {
        return;
      }
      _elements[elementIndex] &= ~(1 << _bitIndexToElementBitIndex(index));
      _shrinkSize();
    }
    void flipBit(int index) {
      if (index < 0) {
        throw new RangeError("Index ($index) must be greater than or equal to 0.");
      }
      int elementIndex = _bitIndexToElementIndex(index);
      if (elementIndex >= _elements.length) {
        _ensureCapacity(elementIndex + 1);
      }
      _elements[elementIndex] ^= (1 << _bitIndexToElementBitIndex(index));
      _usedElements = max(_usedElements, elementIndex + 1);
      _shrinkSize();
    }
  