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

sky/sdk/lib/widgets/popup_menu.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 'dart:async';
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 
 import '../framework/animation/animated_value.dart';
 import '../painting/box_painter.dart';
@@ skipping 49 matching lines @@
       backgroundColor: Grey[50],
       borderRadius: 2.0,
       boxShadow: shadows[level]));
 
     animate(controller.position, (double value) {
       _position = value;
     });
   }
 
   PopupMenuController controller;
-  List<List<UINode>> items;
+  List<UINode> items;
   int level;
 
   void syncFields(PopupMenu source) {
     controller = source.controller;
     items = source.items;
     level = source.level;
     _painter = source._painter;
     super.syncFields(source);
   }
 
   double _position;
   BoxPainter _painter;
 
   double _opacityFor(int i) {
     if (_position == null || _position == 1.0)
       return 1.0;
     double unit = 1.0 / items.length;
     double duration = 1.5 * unit;
     double start = i * unit;
     return math.max(0.0, math.min(1.0, (_position - start) / duration));
   }
 
   UINode build() {
     int i = 0;
-    List<UINode> children = new List.from(items.map((List<UINode> item) {
+    List<UINode> children = new List.from(items.map((UINode item) {
       double opacity = _opacityFor(i);
-      // TODO(abarth): Using |i| for the key here seems wrong.
-      return new PopupMenuItem(key: (i++).toString(),
-                               children: item
+      return new PopupMenuItem(key: '${key}-${item.key}',
+                               child: item,
                                opacity: opacity);
     }));
 
     return new Opacity(
       opacity: math.min(1.0, _position * 3.0),
       child: new ShrinkWrapWidth(
         child: new CustomPaint(
           callback: (sky.Canvas canvas, Size size) {
             double width = math.min(size.width, size.width * (0.5 + _position * 2.0));
             double height = math.min(size.height, size.height * _position * 1.5);
             _painter.paint(canvas, new Rect.fromLTRB(size.width - width, 0.0, width, height));
           },
           child: new Container(
             padding: const EdgeDims.all(8.0),
             child: new Block(children)
           )
         )
       )
     );
   }
 
 }