Set the scrollOffset of a widget's Scrollable ancestor so that the
widget is centered within the scrollable. A future CL will add
support for specifying exactly where the widget appears. The scroll
can be animated by specifying the animation: parameter.

Changed the duration Scrollable.scrollTo() parameter from a Duration
to an AnimationPerformance so that one can configure all aspects of
the animation. The caller may also listen to the animation to schedule
other work while it updates or when its status changes.
complete