decoration param added; warnings eliminated.

[slavap]

Example:

dlg = YYDialog().build(context)
      ..gravity = Gravity.top
      ..width = double.infinity
      ..height = 350
      ..backgroundColor = Colors.teal
      ..decoration = BoxDecoration(
        gradient: LinearGradient(
          // Where the linear gradient begins and ends
          begin: Alignment.topRight,
          end: Alignment.bottomLeft,
          // Add one stop for each color. Stops should increase from 0 to 1
          stops: [0.1, 0.3, 0.5, 0.7, 0.9],
          colors: [
            Colors.teal[800],
            Colors.teal[700],
            Colors.teal[600],
            Colors.teal[500],
            Colors.teal[400],
          ],
        ),
      )
      ..barrierDismissible = false
      ..borderRadius = 0.0

produces the following result: