fix a critical bug

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

 
     abstract class DemoItem
     {
-       public bool isExpanded { get; set; }
+       public virtual bool isExpanded { get; set; }
-       internal ExpansionPanelHeaderBuilder headerBuilder { get; }
+       public virtual ExpansionPanelHeaderBuilder headerBuilder { get; }
 
        internal abstract Widget build();
     }
         internal T value;
         bool _isExpanded = false;
 
-        public new bool isExpanded
+        public override bool isExpanded
-        internal new ExpansionPanelHeaderBuilder headerBuilder {
+        public override ExpansionPanelHeaderBuilder headerBuilder {
             get
             {
                 return (BuildContext context, bool isExpanded) => {

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/gallery/demos.cs

                     documentationUrl: "https://docs.flutter.io/flutter/material/Drawer-class.html",
                     buildRoute: (BuildContext context) => new DrawerDemo()
                 ),
+                new GalleryDemo(
+                    title: "Expansion panels",
+                    subtitle: "List of expanding panels",
+                    icon: GalleryIcons.expand_all,
+                    category: GalleryDemoCategory._kMaterialComponents,
+                    routeName: ExpansionPanelsDemo.routeName,
+                    documentationUrl: "https://docs.flutter.io/flutter/material/ExpansionPanel-class.html",
+                    buildRoute: (BuildContext context) => new ExpansionPanelsDemo()
+                )
             };
 
             return galleryDemos;

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

 using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.CompilerServices;
-using System.Text;
-using Unity.UIWidgets.cupertino;
 using Unity.UIWidgets.foundation;
 using Unity.UIWidgets.rendering;
 using Unity.UIWidgets.ui;
 
         public override void didChangeDependencies() {
             base.didChangeDependencies();
-            bool _didChangeDependencies = true;
+            _didChangeDependencies = true;
         }
 
         public override DiagnosticsNode toDiagnosticsNode(string name = null,