NavigationToolbar constructor

const NavigationToolbar({Key key, Widget leading, Widget middle, Widget trailing, bool centerMiddle: true, double middleSpacing: kMiddleSpacing })

Creates a widget that lays out its children in a manner suitable for a toolbar.

Implementation 

const NavigationToolbar({
  Key key,
  this.leading,
  this.middle,
  this.trailing,
  this.centerMiddle = true,
  this.middleSpacing = kMiddleSpacing,
}) : assert(centerMiddle != null),
     assert(middleSpacing != null),
     super(key: key);