Fix stocks app usage of checkbox

TBR=hixie

Review URL: https://codereview.chromium.org/1188573004.

examples/stocks2/lib/stock_menu.dart

@@ -3,6 +3,7 @@
 // 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';
@@ -22,7 +23,7 @@ class StockMenu extends Component {
 
   UINode build() {
     var checkbox = new Checkbox(
-      checked: this.autorefresh,
+      value: this.autorefresh,
       onChanged: this.onAutorefreshChanged
     );