make overflow dots on iOS horizontal in gallery (#20702)

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

@@ -372,7 +372,11 @@ class _DemoBottomAppBar extends StatelessWidget {
         },
       ),
       new IconButton(
-        icon: const Icon(Icons.more_vert),
+        icon: new Icon(
+          Theme.of(context).platform == TargetPlatform.iOS
+              ? Icons.more_horiz
+              : Icons.more_vert,
+        ),
         onPressed: () {
           Scaffold.of(context).showSnackBar(
             const SnackBar(content: Text('This is a dummy menu action.')),

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

@@ -227,7 +227,11 @@ class _ListDemoState extends State<ListDemo> {
             },
           ),
           new IconButton(
-            icon: const Icon(Icons.more_vert),
+            icon: new Icon(
+              Theme.of(context).platform == TargetPlatform.iOS
+                  ? Icons.more_horiz
+                  : Icons.more_vert,
+            ),
             tooltip: 'Show menu',
             onPressed: _bottomSheet == null ? _showConfigurationSheet : null,
           ),

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

@@ -168,7 +168,11 @@ class _ListDemoState extends State<ReorderableListDemo> {
             },
           ),
           new IconButton(
-            icon: const Icon(Icons.more_vert),
+            icon: new Icon(
+              Theme.of(context).platform == TargetPlatform.iOS
+                  ? Icons.more_horiz
+                  : Icons.more_vert,
+            ),
             tooltip: 'Show menu',
             onPressed: _bottomSheet == null ? _showConfigurationSheet : null,
           ),

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

@@ -52,7 +52,11 @@ class _SearchDemoState extends State<SearchDemo> {
           ),
           new IconButton(
             tooltip: 'More (not implemented)',
-            icon: const Icon(Icons.more_vert),
+            icon: new Icon(
+              Theme.of(context).platform == TargetPlatform.iOS
+                  ? Icons.more_horiz
+                  : Icons.more_vert,
+            ),
             onPressed: () {},
           ),
         ],