anchor.diff:  // It's ok to do this in tests, but you really don't want to do it in
   // production code.
   
  +const String tooltipText = 'TIP';
  +
   void main() {
     testWidgets('Does tooltip end up in the right place - center', (WidgetTester tester) async {
       GlobalKey key = new GlobalKey();
  
positive.code:      *                   *
       *********************/
  
      RenderBox tip = tester.renderObject(find.text('TIP')).parent;
      expect(tip.size.height, equals(100.0));
      expect(tip.localToGlobal(tip.size.topLeft(Point.origin)).y, equals(100.0));
      expect(tip.localToGlobal(tip.size.bottomRight(Point.origin)).y, equals(200.0));
  
negative.code:      *                   * }-10.0 margin
       *********************/
  
      RenderBox tip = tester.renderObject(find.text(tooltipText)).parent;
      expect(tip.size.height, equals(10.0));
      expect(tip.localToGlobal(tip.size.topLeft(Point.origin)).y, equals(310.0));
      expect(tip.localToGlobal(tip.size.bottomRight(Point.origin)).x, equals(790.0));
  