Make it harder to forget itemCount for FixedHeightScrollable

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

Index: sky/sdk/example/demo_launcher/lib/main.dart
diff --git a/sky/sdk/example/demo_launcher/lib/main.dart b/sky/sdk/example/demo_launcher/lib/main.dart
index 458f204c87827736cb8072a01e525823afb4b2cf..139325479f4b99107c7d409f32f0189f448f56bd 100644
--- a/sky/sdk/example/demo_launcher/lib/main.dart
+++ b/sky/sdk/example/demo_launcher/lib/main.dart
@@ -129,9 +129,9 @@ 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(