Fix bugs found by Dart analyzer

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 194107cd1fb9d7b0eae51d0d02778d99ec612d0f..4e9de99266f2d9d6d70c191fd209439fc211000e 100644
--- a/sky/sdk/example/demo_launcher/lib/main.dart
+++ b/sky/sdk/example/demo_launcher/lib/main.dart
@@ -6,14 +6,11 @@ import 'dart:sky';
 
 import 'package:sky/mojo/activity.dart' as activity;
 import 'package:sky/mojo/asset_bundle.dart';
-import 'package:sky/mojo/shell.dart' as shell;
 import 'package:sky/painting/box_painter.dart';
 import 'package:sky/theme/colors.dart' as colors;
 import 'package:sky/theme/typography.dart' as typography;
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/card.dart';
-import 'package:sky/widgets/fixed_height_scrollable.dart';
-import 'package:sky/widgets/flat_button.dart';
 import 'package:sky/widgets/material.dart';
 import 'package:sky/widgets/scaffold.dart';
 import 'package:sky/widgets/task_description.dart';
@@ -66,7 +63,7 @@ class SkyDemo {
   SkyDemo({ this.name, this.href, this.bundle, this.description, this.textTheme, this.decoration });
 }
 
-List<Widget> demos = [
+List<SkyDemo> demos = [
   new SkyDemo(
     name: 'Stocks',
     href: '../../stocks/lib/main.dart',