anchor.diff:          && darkColor != null
           && textColor != null
           && style != null
  +        && margin != null
           && _position != null
           && _position.isFinite
           && _opacity != null
           && _opacity >= 0.0
  -        && _opacity <= 1.0
  -        && margin != null);
  +        && _opacity <= 1.0);
       return true;
     }
   
  
positive.code:         a.darkColor,
          a.textColor,
          a.style,
          a._position,
          a._opacity * (1.0 - t).clamp(0.0, 1.0),
          a.margin * t,
        );
      }
      return new FlutterLogoDecoration._(
  
negative.code:   // set the internal _position property.
    final FlutterLogoStyle style;
  
    /// How far to inset the logo from the edge of the container.
    final EdgeInsets margin;
  
    // The following are set when lerping, to represent states that can't be
    // represented by the constructor.
    final double _position; // -1.0 for stacked, 1.0 for horizontal, 0.0 for no logo
    final double _opacity; // 0.0 .. 1.0
  
    bool get _inTransition => _opacity != 1.0 || (_position != -1.0 && _position != 0.0 && _position != 1.0);
  
  