Fix bugs found by Dart analyzer

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: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';
 import 'package:sky/widgets/theme.dart';
 import 'package:sky/widgets/tool_bar.dart';
 import 'package:sky/widgets/scrollable_list.dart';
 
 AssetBundle _initBundle() {
   if (rootBundle != null)
     return rootBundle;
@@ skipping 32 matching lines @@
 class SkyDemo {
   String name;
   String href;
   String bundle;
   String description;
   typography.TextTheme textTheme;
   BoxDecoration decoration;
   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',
     bundle: 'stocks.skyx',
     description: 'Multi-screen app with scrolling list',
     textTheme: typography.black,
     decoration: new BoxDecoration(
       backgroundImage: new BackgroundImage(
         image: _bundle.loadImage('assets/stocks_thumbnail.png'),
         fit: BackgroundFit.cover
@@ skipping 111 matching lines @@
           )
         )
       )
     );
   }
 }
 
 void main() {
   runApp(new SkyHome());
 }