fix nested_scroll_view bug

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/demo/material/tabs_demo.cs

 {
     static class TabsDemoUtils
     {
-        public static readonly string _kGalleryAssetsPackage = "flutter_gallery_assets";
+        public static readonly string _kGalleryAssetsPackage = "gallery/";
 
         public static readonly Dictionary<_TabPage, List<_CardData>> _allPages =
             new Dictionary<_TabPage, List<_CardData>>

com.unity.uiwidgets/Runtime/widgets/nested_scroll_view.cs

                             : new ClampingScrollPhysics(),
                         controller: _coordinator._outerController,
                         slivers: widget._buildSlivers(
-                            context,
+                            _context,
                             _coordinator._innerController,
                             _lastHasScrolledBody
                         ),