anchor.diff:        value: 1.0,
         vsync: this,
       );
  +
  +    _frontOpacity =
  +      new Tween<double>(begin: 0.2, end: 1.0).animate(
  +        new CurvedAnimation(
  +          parent: _controller,
  +          curve: const Interval(0.0, 0.4, curve: Curves.easeInOut),
  +        ),
  +      );
     }
   
     @override
  
positive.code: class _BackdropState extends State<Backdrop> with SingleTickerProviderStateMixin {
    final GlobalKey _backdropKey = new GlobalKey(debugLabel: 'Backdrop');
    AnimationController _controller;
  
    @override
    void initState() {
  
negative.code:             child: new _IgnorePointerWhileStatusIsNot(
                AnimationStatus.completed,
                controller: _controller,
                child: new FadeTransition(
                  opacity: _frontOpacity,
                  child: widget.frontLayer,
                ),
              ),
            ),
          ),
  