Add a basic popup menu widget

Currently this widget is demoed in widgets-fn, but I'll move it into stocks-fn
soon.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1017873002

examples/widgets-fn/widgets_app.dart

@@ -4,9 +4,37 @@
 
 import '../../framework/fn.dart';
 import '../../framework/components/button.dart';
+import '../../framework/components/popup_menu.dart';
+import '../../framework/components/popup_menu_item.dart';
 
 class WidgetsApp extends App {
+  static final Style _menuStyle = new Style('''
+    position: absolute;
+    top: 200px;
+    left: 200px;''');
+
   Node build() {
-    return new Button(content: new Text('Go'), level: 1);
+    return new Container(
+      children: [
+        new Button(key: 'Go', content: new Text('Go'), level: 1),
+        new Button(key: 'Back', content: new Text('Back'), level: 3),
+        new Container(
+          style: _menuStyle,
+          children: [
+            new PopupMenu(
+              children: [
+                  new PopupMenuItem(key: '1', children: [new Text('People & options')]),
+                  new PopupMenuItem(key: '2', children: [new Text('New group conversation')]),
+                  new PopupMenuItem(key: '3', children: [new Text('Turn history off')]),
+                  new PopupMenuItem(key: '4', children: [new Text('Archive')]),
+                  new PopupMenuItem(key: '5', children: [new Text('Delete')]),
+                  new PopupMenuItem(key: '6', children: [new Text('Un-merge SMS')]),
+                  new PopupMenuItem(key: '7', children: [new Text('Help & feeback')]),
+              ],
+              level: 4),
+            ]
+          )
+        ]
+      );
   }
 }