Upgraded to Flutter 1.9.0

packages/zefyr/lib/src/widgets/selection.dart

     // Make sure the GestureDetector is big enough to be easily interactive.
     final Rect interactiveRect = handleRect.expandToInclude(
       Rect.fromCircle(
-          center: handleRect.center, radius: kMinInteractiveSize / 2),
+          center: handleRect.center, radius: kMinInteractiveDimension / 2),
     );
     final RelativeRect padding = RelativeRect.fromLTRB(
       math.max((interactiveRect.width - handleRect.width) / 2, 0),