anchor.diff:                      top: 0.0,
                       child: new Tooltip(
                         key: key,
  -                      message: 'TIP',
  +                      message: tooltipText,
                         height: 20.0,
                         padding: const EdgeInsets.all(5.0),
                         verticalOffset: 20.0,
  
positive.code:      *                   * }-10.0 margin
       *********************/
  
      RenderBox tip = tester.renderObject(find.text('TIP')).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));
  
negative.code: // 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();
  