Scroll this widget by the given scroll delta.
If a non-null duration is provided, the widget will animate to the new
scroll offset over the given duration with the given curve.
Source
Future<Null> scrollBy(double scrollDelta, {
Duration duration,
Curve curve: Curves.ease,
DragUpdateDetails details
}) {
double newScrollOffset = scrollBehavior.applyCurve(virtualScrollOffset, scrollDelta);
return scrollTo(newScrollOffset, duration: duration, curve: curve, details: details);
}