Make the popup menu work again.

sky/sdk/lib/widgets/widget.dart

Index: sky/sdk/lib/widgets/widget.dart
diff --git a/sky/sdk/lib/widgets/widget.dart b/sky/sdk/lib/widgets/widget.dart
index 11fb763a7db1f1986c907bfe9a7fb4ae705b54b5..a83b2c77cc56e796feaa08051427bb1b5c49fa18 100644
--- a/sky/sdk/lib/widgets/widget.dart
+++ b/sky/sdk/lib/widgets/widget.dart
@@ -21,8 +21,7 @@ final bool _shouldLogRenderDuration = false;
 // can be sync'd.
 abstract class Widget {
 
-  Widget({ String key }) {
-    _key = key != null ? key : runtimeType.toString();
+  Widget({ String key }) : _key = key {
     assert(this is AbstractWidgetRoot || this is App || _inRenderDirtyComponents); // you should not build the UI tree ahead of time, build it only during build()
   }
 
@@ -84,8 +83,6 @@ abstract class Widget {
   // Component._retainStatefulNodeIfPossible() calls syncFields().
   bool _retainStatefulNodeIfPossible(Widget old) => false;
 
-  bool get interchangeable => false; // if true, then keys can be duplicated
-
   void _sync(Widget old, dynamic slot);
   // 'slot' is the identifier that the parent RenderObjectWrapper uses to know
   // where to put this descendant
@@ -154,7 +151,11 @@ abstract class Widget {
     return node;
   }
 
-  String toString() => '$runtimeType($key)';
+  String toString() {
+    if (key == null)
+      return '$runtimeType(unkeyed)';
+    return '$runtimeType("$key")';
+  }
 
 }
 
@@ -503,8 +504,6 @@ abstract class RenderObjectWrapper extends Widget {
       _ancestor = findAncestor(RenderObjectWrapper);
       if (_ancestor is RenderObjectWrapper)
         _ancestor.insert(this, slot);
-      else
-        print("$this has no ancestor");
     } else {
       _root = old.root;
       _ancestor = old._ancestor;
@@ -630,11 +629,11 @@ abstract class MultiChildRenderObjectWrapper extends RenderObjectWrapper {
     var idSet = new HashSet<String>();
     for (var child in children) {
       assert(child != null);
-      if (child.interchangeable)
+      if (child.key == null)
         continue; // when these nodes are reordered, we just reassign the data
 
       if (!idSet.add(child.key)) {
-        throw '''If multiple non-interchangeable nodes exist as children of another node, they must have unique keys. Duplicate: "${child.key}"''';
+        throw '''If multiple keyed nodes exist as children of another node, they must have unique keys. $this has duplicate child key "${child.key}".''';
       }
     }
     return false;
@@ -702,13 +701,13 @@ abstract class MultiChildRenderObjectWrapper extends RenderObjectWrapper {
       oldNodeIdMap = new HashMap<String, Widget>();
       for (int i = oldStartIndex; i < oldEndIndex; i++) {
         var node = oldChildren[i];
-        if (!node.interchangeable)
+        if (node.key != null)
           oldNodeIdMap.putIfAbsent(node.key, () => node);
       }
     }
 
     bool searchForOldNode() {
-      if (currentNode.interchangeable)
+      if (currentNode.key == null)
         return false; // never re-order these nodes
 
       ensureOldIdMap();