Make it possible to detect PopupMenuItem presses

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:math' as math;
 import 'dart:sky' as sky;
 
 import 'package:sky/animation/animation_performance.dart';
 import 'package:sky/painting/box_painter.dart';
 import 'package:sky/theme/colors.dart';
@@ skipping 103 matching lines @@
       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.value - start) / duration));
   }
 
   Widget build() {
     int i = 0;
     List<Widget> children = new List.from(items.map((Widget item) {
-      double opacity = _opacityFor(i);
-      return new PopupMenuItem(child: item, opacity: opacity);
+      return new Opacity(opacity: _opacityFor(i++), child: item);
     }));
 
     return new Opacity(
       opacity: math.min(1.0, _position.value * 3.0),
       child: new Container(
         margin: new EdgeDims.all(_kMenuMargin),
         child: new CustomPaint(
           callback: (sky.Canvas canvas, Size size) {
             double width = math.min(size.width, size.width * (0.5 + _position.value * 2.0));
             double height = math.min(size.height, size.height * _position.value * 1.5);
@@ skipping 16 matching lines @@
                 )
               )
             )
           )
         )
       )
     );
   }
 
 }