Make PopupMenuItem take a single child instead of an implicitly-flex list of children.

sky/sdk/lib/widgets/popup_menu_item.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 'basic.dart';
 import 'ink_well.dart';
 
 class PopupMenuItem extends Component {
-  PopupMenuItem({ String key, this.children, this.opacity}) : super(key: key);
+  PopupMenuItem({ String key, this.child, this.opacity}) : super(key: key);
 
-  final List<UINode> children;
+  final UINode child;
   final double opacity;
 
   UINode build() {
     return new Opacity(
       opacity: opacity,
       child: new InkWell(
         child: new Container(
           constraints: const BoxConstraints(minWidth: 112.0),
           padding: const EdgeDims.all(16.0),
-          child: new Flex(children)
+          child: child
         )
       )
     );
   }
 }