AnimatedDefaultTextStyle({Key key, @required Widget child, @required TextStyle style, Curve curve: Curves.linear, @required Duration duration })

Creates a widget that animates the default text style implicitly.

The child, style, curve, and duration arguments must not be null.

Source 

AnimatedDefaultTextStyle({
  Key key,
  @required this.child,
  @required this.style,
  Curve curve: Curves.linear,
  @required Duration duration,
}) : super(key: key, curve: curve, duration: duration) {
  assert(style != null);
  assert(child != null);
}