Remove unneeded import and fix analyzer warnings

sky/examples/stocks2/lib/stock_menu.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 'package:sky/widgets/checkbox.dart';
-import 'package:sky/widgets/toggleable.dart';
 import 'package:sky/widgets/popup_menu.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/framework/theme/view_configuration.dart';
 
 class StockMenu extends Component {
 
   StockMenu({
     Object key,
     this.controller,
     this.autorefresh: false,
@@ skipping 18 matching lines @@
           [new Text('Remove stock')],
           [new FlexExpandingChild(new Text('Autorefresh')), checkbox],
         ],
         level: 4
       ),
       right: 8.0,
       top: 8.0 + kStatusBarHeight
     );
   }
 }