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

TBR=abarth

Review URL: https://codereview.chromium.org/1187463013.

examples/stocks2/lib/stock_menu.dart

@@ -30,9 +30,9 @@ class StockMenu extends Component {
       child: new PopupMenu(
         controller: controller,
         items: [
-          [new Text('Add stock')],
-          [new Text('Remove stock')],
-          [new Flexible(child: new Text('Autorefresh')), checkbox],
+          new Text('Add stock'),
+          new Text('Remove stock'),
+          new Flex([new Flexible(child: new Text('Autorefresh')), checkbox]),
         ],
         level: 4
       ),

sdk/lib/widgets/popup_menu.dart

@@ -67,7 +67,7 @@ class PopupMenu extends AnimatedComponent {
   }
 
   PopupMenuController controller;
-  List<List<UINode>> items;
+  List<UINode> items;
   int level;
 
   void syncFields(PopupMenu source) {
@@ -92,11 +92,10 @@ class PopupMenu extends AnimatedComponent {
 
   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);
     }));
 

sdk/lib/widgets/popup_menu_item.dart

@@ -6,9 +6,9 @@ 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() {
@@ -18,7 +18,7 @@ class PopupMenuItem extends Component {
         child: new Container(
           constraints: const BoxConstraints(minWidth: 112.0),
           padding: const EdgeDims.all(16.0),
-          child: new Flex(children)
+          child: child
         )
       )
     );