[Gallery] Fix directory structure (#312)

gallery/lib/demos/material/list_demo.dart

@@ -0,0 +1,51 @@
+// Copyright 2019 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+
+import 'package:gallery/l10n/gallery_localizations.dart';
+
+// BEGIN listDemo
+
+enum ListDemoType {
+  oneLine,
+  twoLine,
+}
+
+class ListDemo extends StatelessWidget {
+  const ListDemo({Key key, this.type}) : super(key: key);
+
+  final ListDemoType type;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        automaticallyImplyLeading: false,
+        title: Text(GalleryLocalizations.of(context).demoListsTitle),
+      ),
+      body: Scrollbar(
+        child: ListView(
+          padding: EdgeInsets.symmetric(vertical: 8),
+          children: [
+            for (int index = 1; index < 21; index++)
+              ListTile(
+                leading: ExcludeSemantics(
+                  child: CircleAvatar(child: Text('$index')),
+                ),
+                title: Text(
+                  GalleryLocalizations.of(context).demoBottomSheetItem(index),
+                ),
+                subtitle: type == ListDemoType.twoLine
+                    ? Text(GalleryLocalizations.of(context).demoListsSecondary)
+                    : null,
+              ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+// END