1. override
void debugPaintSize(PaintingContext context, Offset offset)

In debug mode, paints a border around this render box.

Called for every RenderBox when debugPaintSizeEnabled is true.

Source 

@override
void debugPaintSize(PaintingContext context, Offset offset) {
  super.debugPaintSize(context, offset);
  assert(() {
    Paint paint;
    if (child == null || child.size.isEmpty) {
      paint = new Paint()
        ..color = debugPaintSpacingColor;
      context.canvas.drawRect(offset & size, paint);
    }
    return true;
  });
}