Make no-op buttons more accessible by showing a SnackBar when they're pressed (#16284)

* Remove explicit child nodes

* Remove semantics on no-op menu buttons

* Add a dummy action to the overflow bar

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

@@ -181,7 +181,6 @@ class _BottomAppBarDemoState extends State<BottomAppBarDemo> {
         new Semantics(
           label: 'Set Bottom App Bar color to ${colorToName[color]}',
           container: true,
-          explicitChildNodes: false,
           child: new Row(children: <Widget> [
             new Radio<Color>(
               value: color,
@@ -311,34 +310,21 @@ class _DemoBottomAppBar extends StatelessWidget {
       );
     }
     rowContents.addAll(<Widget> [
-      // Generally, an icon button does not need to be wrapped in a Semantics object.
-      // When the button has a null onPressed callback, it will be disabled by default.
-      //
-      // However, for the purposes of this demo, we don't want the button to appear disabled.
-      // To achieve this we use a no-op callback instead of a null callback. This allows
-      // the buttons to appear active, but perform no actions.
-      //
-      // To tell the accessibility service that the callback is a no-op, the Semantics widget
-      // wraps these buttons with `enabled: false`.
-      new Semantics(
-        label: 'Search',
-        container: true,
-        explicitChildNodes: false,
-        enabled: false,
-        child: new IconButton(
-          icon: const Icon(Icons.search),
-          onPressed: () {},
-        ),
+      new IconButton(
+        icon: const Icon(Icons.search),
+        onPressed: () {
+          Scaffold.of(context).showSnackBar(
+            const SnackBar(content: const Text('This is a dummy search action.')),
+          );
+        },
       ),
-      new Semantics(
-        label: 'Show more',
-        container: true,
-        explicitChildNodes: false,
-        enabled: false,
-        child: new IconButton(
-          icon: const Icon(Icons.more_vert),
-          onPressed: () {},
-        ),
+      new IconButton(
+        icon: const Icon(Icons.more_vert),
+        onPressed: () {
+          Scaffold.of(context).showSnackBar(
+            const SnackBar(content: const Text('This is a dummy menu action.')),
+          );
+        },
       ),
     ]);
     return new Row(