title: Flutter Animation controller Invalid reference to 'this' expression
body: <p>I am using flutter spinkkit plugin issue is its showing an error on vsync this. <code>Invalid reference to 'this' expression.</code></p>
  <p>final spinkit = SpinKitSquareCircle(
  color: kPrimaryColor,
  size: 50.0,
  controller:  AnimationController(vsync: this, duration: const Duration(milliseconds: 1200)),
  );</p>
  <p>So somewhere I read that I need to place it inside init state</p>
  <p>So if I place this in init state its showing the error Undefined name 'spinkit'. where I am using spinner.</p>