anchor.diff:          scrollDirection: scrollDirection,
           scrollOffset: viewportOffset,
           child: new Container(
  -          padding: padding,
  +          padding: _crossAxisPadding,
             child: new Block(items, direction: blockDirection)
           )
         )
  
positive.code:       _updateScrollOffset();
      }
  
      int itemShowIndex = 0;
      int itemShowCount = 0;
      Offset viewportOffset = Offset.zero;
      if (_containerExtent != null && _containerExtent > 0.0) {
        if (scrollOffset < 0.0) {
          double visibleHeight = _containerExtent + scrollOffset;
          itemShowCount = (visibleHeight / itemExtent).round() + 1;
          viewportOffset = _toOffset(scrollOffset);
        } else {
          itemShowCount = (_containerExtent / itemExtent).ceil();
          double alignmentDelta = -scrollOffset % itemExtent;
          double drawStart;
          if (alignmentDelta != 0.0) {
            alignmentDelta -= itemExtent;
            itemShowCount += 1;
            drawStart = scrollOffset + alignmentDelta;
            viewportOffset = _toOffset(-alignmentDelta);
          } else {
            drawStart = scrollOffset;
          }
          itemShowIndex = math.max(0, (drawStart / itemExtent).floor());
        }
  
negative.code:     });
    }
  
    double get _leadingPadding {
      if (scrollDirection == ScrollDirection.vertical)
        return padding.top;
      return padding.left;
    }
  
    double get _trailingPadding {
      if (scrollDirection == ScrollDirection.vertical)
        return padding.bottom;
      return padding.right;
    }
  
    EdgeDims get _crossAxisPadding {
      if (padding == null)
        return null;
      if (scrollDirection == ScrollDirection.vertical)
        return new EdgeDims.only(left: padding.left, right: padding.right);
      return new EdgeDims.only(top: padding.top, bottom: padding.bottom);
    }
  
    void _updateContentsExtent() {
      double contentsExtent = itemExtent * itemCount;
      if (padding != null)
        contentsExtent += _leadingPadding + _trailingPadding;
      scrollBehavior.contentsSize = contentsExtent;
    }
  
  