Placeholder constructor

const Placeholder({Key key, Color color: const Color(0xFF455A64), double strokeWidth: 2.0, double fallbackWidth: 400.0, double fallbackHeight: 400.0 })

Creates a widget which draws a box.

Implementation 

const Placeholder({
  Key key,
  this.color = const Color(0xFF455A64), // Blue Grey 700
  this.strokeWidth = 2.0,
  this.fallbackWidth = 400.0,
  this.fallbackHeight = 400.0,
}) : super(key: key);