Make it harder to forget itemCount for FixedHeightScrollable

sky/sdk/example/demo_launcher/lib/main.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:sky';
 
 import 'package:mojom/intents/intents.mojom.dart';
 import 'package:sky/mojo/asset_bundle.dart';
 import 'package:sky/mojo/shell.dart' as shell;
 import 'package:sky/painting/box_painter.dart';
@@ skipping 111 matching lines @@
   ),
 
   // TODO(jackson): This doesn't seem to be working
   // new SkyDemo('Licenses', 'LICENSES.sky'),
 ];
 
 const double kCardHeight = 120.0;
 const EdgeDims kListPadding = const EdgeDims.all(4.0);
 
 class DemoList extends FixedHeightScrollable {
-  DemoList({ String key }) : super(key: key, itemHeight: kCardHeight, padding: kListPadding) {
-    itemCount = demos.length;
-  }
+  DemoList({ String key }) : super(key: key, itemHeight: kCardHeight, padding: kListPadding);
+
+  int get itemCount => demos.length;
 
   Widget buildDemo(SkyDemo demo) {
     return new Listener(
       key: demo.name,
       onGestureTap: (_) => launch(demo.href, demo.bundle),
       child: new Container(
         height: kCardHeight,
         child: new Card(
           child: new Flex([
             new Flexible(
@@ skipping 40 matching lines @@
           child: new DemoList()
         )
       )
     );
   }
 }
 
 void main() {
   runApp(new SkyHome());
 }