Gallery a11y fix: give the categories and demos pages "route" scope (#17516)

examples/flutter_gallery/lib/gallery/home.dart

@@ -109,7 +109,12 @@ class _CategoriesPage extends StatelessWidget {
     final List<GalleryDemoCategory> categoriesList = categories.toList();
     final int columnCount = (MediaQuery.of(context).orientation == Orientation.portrait) ? 2 : 3;
 
-    return new SingleChildScrollView(
+    return new Semantics(
+      scopesRoute: true,
+      namesRoute: true,
+      label: 'categories',
+      explicitChildNodes: true,
+      child: new SingleChildScrollView(
         key: const PageStorageKey<String>('categories'),
         child: new LayoutBuilder(
           builder: (BuildContext context, BoxConstraints constraints) {
@@ -151,6 +156,7 @@ class _CategoriesPage extends StatelessWidget {
             );
           },
         ),
+      ),
     );
   }
 }
@@ -240,6 +246,11 @@ class _DemosPage extends StatelessWidget {
   Widget build(BuildContext context) {
     return new KeyedSubtree(
       key: const ValueKey<String>('GalleryDemoList'), // So the tests can find this ListView
+      child: new Semantics(
+        scopesRoute: true,
+        namesRoute: true,
+        label: category.name,
+        explicitChildNodes: true,
         child: new ListView(
           key: new PageStorageKey<String>(category.name),
           padding: const EdgeInsets.only(top: 8.0),
@@ -247,6 +258,7 @@ class _DemosPage extends StatelessWidget {
             return new _DemoItem(demo: demo);
           }).toList(),
         ),
+      ),
     );
   }
 }