[Effen] Use the checkbox widget in the stocks app.

- add a checkbox to the stock app, so that we're testing the checkbox widget
  (it's not currently wired up to anything, that can come later)
- make InkSplash use FlexContainer so that we can use flex in the popup menu items
- make effen's Text be more similar to Image and Container, so that it can be styled
- make layout.dart's RenderCSSText correctly support being styled
- also fixes a bug with the stock list where we were rendering one too few a row when scrolling
- check in the code to dump the DOM so I don't have to keep remembering how to do this

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1134163003

examples/stocks/lib/stock_app.dart

@@ -21,6 +21,11 @@ import 'stock_data.dart';
 import 'stock_list.dart';
 import 'stock_menu.dart';
 
+import 'dart:async';
+import 'package:sky/framework/layout.dart';
+
+const bool debug = false; // set to true to dump the DOM for debugging purposes
+
 class StocksApp extends App {
   DrawerController _drawerController = new DrawerController();
   PopupMenuController _menuController;
@@ -39,6 +44,8 @@ class StocksApp extends App {
   String _searchQuery;
 
   StocksApp() : super() {
+    if (debug)
+      new Timer(new Duration(seconds: 1), dumpState);
     new StockDataFetcher((StockData data) {
       setState(() {
         data.appendTo(_stocks);

examples/stocks/lib/stock_menu.dart

@@ -2,8 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:sky/framework/components/popup_menu.dart';
 import 'package:sky/framework/fn.dart';
+import 'package:sky/framework/layout.dart';
+import 'package:sky/framework/components/popup_menu.dart';
+import 'package:sky/framework/components/checkbox.dart';
 import 'package:sky/framework/theme/view_configuration.dart';
 
 class StockMenu extends Component {
@@ -14,16 +16,26 @@ class StockMenu extends Component {
 
   PopupMenuController controller;
 
-  StockMenu({Object key, this.controller}) : super(key: key);
+  StockMenu({Object key, this.controller, this.autorefresh: false, this.onAutorefreshChanged}) : super(key: key);
+
+  final bool autorefresh;
+  final ValueChanged onAutorefreshChanged;
+
+  static FlexBoxParentData _flex1 = new FlexBoxParentData()..flex = 1;
 
   UINode build() {
+    var checkbox = new Checkbox(
+      checked: this.autorefresh,
+      onChanged: this.onAutorefreshChanged
+    );
+
     return new StyleNode(
       new PopupMenu(
         controller: controller,
         items: [
           [new Text('Add stock')],
           [new Text('Remove stock')],
-          [new Text('Help & feedback')],
+          [new ParentDataNode(new Text('Autorefresh'), _flex1), checkbox],
         ],
         level: 4),
         _style