a11y fix for list_demo.dart (#11142)

* a11y fix for list_demo.dart

* fix semantics of persistant bottom sheet

examples/flutter_gallery/lib/demo/material/list_demo.dart

@@ -58,7 +58,8 @@ class _ListDemoState extends State<ListDemo> {
           shrinkWrap: true,
           primary: false,
           children: <Widget>[
-            new ListTile(
+            new MergeSemantics(
+              child: new ListTile(
                 dense: true,
                 title: const Text('One-line'),
                 trailing: new Radio<_MaterialListType>(
@@ -67,7 +68,9 @@ class _ListDemoState extends State<ListDemo> {
                   onChanged: changeItemType,
                 )
               ),
-            new ListTile(
+            ),
+            new MergeSemantics(
+              child: new ListTile(
                 dense: true,
                 title: const Text('Two-line'),
                 trailing: new Radio<_MaterialListType>(
@@ -76,7 +79,9 @@ class _ListDemoState extends State<ListDemo> {
                   onChanged: changeItemType,
                 )
               ),
-            new ListTile(
+            ),
+            new MergeSemantics(
+              child: new ListTile(
                 dense: true,
                 title: const Text('Three-line'),
                 trailing: new Radio<_MaterialListType>(
@@ -85,7 +90,9 @@ class _ListDemoState extends State<ListDemo> {
                   onChanged: changeItemType,
                 ),
               ),
-            new ListTile(
+            ),
+            new MergeSemantics(
+              child: new ListTile(
                 dense: true,
                 title: const Text('Show avatar'),
                 trailing: new Checkbox(
@@ -98,7 +105,9 @@ class _ListDemoState extends State<ListDemo> {
                   },
                 ),
               ),
-            new ListTile(
+            ),
+            new MergeSemantics(
+              child: new ListTile(
                 dense: true,
                 title: const Text('Show icon'),
                 trailing: new Checkbox(
@@ -111,7 +120,9 @@ class _ListDemoState extends State<ListDemo> {
                   },
                 ),
               ),
-            new ListTile(
+            ),
+            new MergeSemantics(
+              child: new ListTile(
                 dense: true,
                 title: const Text('Show dividers'),
                 trailing: new Checkbox(
@@ -124,7 +135,9 @@ class _ListDemoState extends State<ListDemo> {
                   },
                 ),
               ),
-            new ListTile(
+            ),
+            new MergeSemantics(
+              child: new ListTile(
                 dense: true,
                 title: const Text('Dense layout'),
                 trailing: new Checkbox(
@@ -137,6 +150,7 @@ class _ListDemoState extends State<ListDemo> {
                   },
                 ),
               ),
+            ),
           ],
         ),
       );
@@ -164,13 +178,15 @@ class _ListDemoState extends State<ListDemo> {
         "Even more additional list item information appears on line three.",
       );
     }
-    return new ListTile(
+    return new MergeSemantics(
+      child: new ListTile(
         isThreeLine: _itemType == _MaterialListType.threeLine,
         dense: _dense,
-      leading: _showAvatars ? new CircleAvatar(child: new Text(item)) : null,
+        leading: _showAvatars ? new ExcludeSemantics(child: new CircleAvatar(child: new Text(item))) : null,
         title: new Text('This item represents $item.'),
         subtitle: secondary,
         trailing: _showIcons ? new Icon(Icons.info, color: Theme.of(context).disabledColor) : null,
+      ),
     );
   }