NestedScrollView constructor
Creates a nested scroll view.
The reverse, headerSliverBuilder, and body arguments must not be
null.
Implementation
const NestedScrollView({
  Key key,
  this.controller,
  this.scrollDirection = Axis.vertical,
  this.reverse = false,
  this.physics,
  @required this.headerSliverBuilder,
  @required this.body,
}) : assert(scrollDirection != null),
     assert(reverse != null),
     assert(headerSliverBuilder != null),
     assert(body != null),
     super(key: key);