fix errors

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/demo/cupertino/cupertino_segmented_control_demo.cs

                   width: 500,
                   child: new Padding(
                     padding: EdgeInsets.all(16.0f),
-                    child: CupertinoSlidingSegmentedControl<int>(
+                    child: new CupertinoSlidingSegmentedControl<int>(
                       children: children,
                       onValueChanged: onValueChanged,
                       groupValue: currentSegment