Position the popup menu in stocks2 correctly

This CL pulled an implementation of position: absolute in RenderStackContainer.

R=eseidel@chromium.org, ianh@google.com

Review URL: https://codereview.chromium.org/1166363002.

examples/stocks2/lib/stock_menu.dart

@@ -8,11 +8,6 @@ import 'package:sky/framework/components2/checkbox.dart';
 import 'package:sky/framework/theme/view_configuration.dart';
 
 class StockMenu extends Component {
-  // static final Style _style = new Style('''
-  //   position: absolute;
-  //   right: 8px;
-  //   top: ${8 + kStatusBarHeight}px;''');
-
   PopupMenuController controller;
 
   StockMenu({Object key, this.controller, this.autorefresh: false, this.onAutorefreshChanged}) : super(key: key);
@@ -26,7 +21,8 @@ class StockMenu extends Component {
       onChanged: this.onAutorefreshChanged
     );
 
-    return new PopupMenu(
+    return new StackPositionedChild(
+      new PopupMenu(
         controller: controller,
         items: [
           [new Text('Add stock')],
@@ -34,6 +30,9 @@ class StockMenu extends Component {
           // [new FlexExpandingChild(new Text('Autorefresh')), checkbox],
         ],
         level: 4
+      ),
+      right: 8.0,
+      top: 8.0 + kStatusBarHeight
     );
   }
 }