We now use the `@required` annotation to encourage developers to
explicitly set onPressed and onChanged callbacks to null when that would
disable the widget.
Fixes#287
Although beautiful, having a continuous animation on the front page of the
gallery gives the impression that Flutter burns battery and makes phones hot.
This patch replaces the animation with solid purple. However, we might
eventually want to use a static image.
This demo burns battery continuously even when there's no change to the visual
appearance. This patch removes this demo to avoid giving the impression that
Flutter burns battery.
Related to #4120
