ImplicitlyAnimatedWidget({Key key, Curve curve: Curves.linear, @required Duration duration })

Initializes fields for subclasses.

The curve and duration arguments must not be null.

Source 

ImplicitlyAnimatedWidget({
  Key key,
  this.curve: Curves.linear,
  @required this.duration
}) : super(key: key) {
  assert(curve != null);
  assert(duration != null);
}