TwoLevelList fails to restore sublist expanded state

packages/flutter/lib/src/material/two_level_list.dart

@@ -154,11 +154,14 @@ class _TwoLevelSublistState extends State<TwoLevelSublist> {
 }
 
 class TwoLevelList extends StatelessWidget {
-  TwoLevelList({ Key key, this.items, this.type: MaterialListType.twoLine }) : super(key: key);
+  TwoLevelList({ Key key, this.scrollableKey, this.items, this.type: MaterialListType.twoLine }) : super(key: key);
 
   final List<Widget> items;
   final MaterialListType type;
+  final Key scrollableKey;
 
   @override
-  Widget build(BuildContext context) => new Block(children: items);
+  Widget build(BuildContext context) {
+    return new Block(children: KeyedSubtree.ensureUniqueKeysForList(items), scrollableKey: scrollableKey);
+  }
 }

packages/flutter/lib/src/widgets/basic.dart

@@ -7,6 +7,7 @@ import 'dart:ui' as ui show Image;
 import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
 
+import 'debug.dart';
 import 'framework.dart';
 
 export 'package:flutter/animation.dart';
@@ -2575,6 +2576,27 @@ class KeyedSubtree extends StatelessWidget {
   /// The widget below this widget in the tree.
   final Widget child;
 
+  /// Wrap each item in a KeyedSubtree whose key is based on the item's existing key or
+  /// its list index + baseIndex.
+  static List<Widget> ensureUniqueKeysForList(Iterable<Widget> items, { int baseIndex: 0 }) {
+    if (items == null || items.isEmpty)
+      return items;
+
+    List<Widget> itemsWithUniqueKeys = <Widget>[];
+    int itemIndex = baseIndex;
+    for(Widget item in items) {
+      itemsWithUniqueKeys.add(new KeyedSubtree(
+        key: item.key != null ? new ValueKey<Key>(item.key) : new ValueKey<int>(itemIndex),
+        child: item
+      ));
+      itemIndex += 1;
+    }
+
+    assert(!debugItemsHaveDuplicateKeys(itemsWithUniqueKeys));
+    return items;
+  }
+
+
   @override
   Widget build(BuildContext context) => child;
 }

packages/flutter/lib/src/widgets/debug.dart

@@ -25,22 +25,39 @@ bool debugCheckHasMediaQuery(BuildContext context) {
   return true;
 }
 
-bool debugHasDuplicateKeys(Widget parent, Iterable<Widget> children) {
+Key _firstNonUniqueKey(Iterable<Widget> widgets) {
+  Set<Key> keySet = new HashSet<Key>();
+  for (Widget widget in widgets) {
+    assert(widget != null);
+    if (widget.key == null)
+      continue;
+    if (!keySet.add(widget.key))
+      return widget.key;
+  }
+  return null;
+}
+
+bool debugChildrenHaveDuplicateKeys(Widget parent, Iterable<Widget> children) {
   assert(() {
-    Set<Key> keySet = new HashSet<Key>();
-    for (Widget child in children) {
-      assert(child != null);
-      if (child.key == null)
-        continue;
-      if (!keySet.add(child.key)) {
-        throw new FlutterError(
-          'Duplicate keys found.\n'
-          'If multiple keyed nodes exist as children of another node, they must have unique keys.\n'
-          '$parent has multiple children with key "${child.key}".'
-        );
-      }
+    final Key nonUniqueKey = _firstNonUniqueKey(children);
+    if (nonUniqueKey != null) {
+      throw new FlutterError(
+        'Duplicate keys found.\n'
+        'If multiple keyed nodes exist as children of another node, they must have unique keys.\n'
+        '$parent has multiple children with key $nonUniqueKey.'
+      );
     }
     return true;
   });
   return false;
 }
+
+bool debugItemsHaveDuplicateKeys(Iterable<Widget> items) {
+  assert(() {
+    final Key nonUniqueKey = _firstNonUniqueKey(items);
+    if (nonUniqueKey != null)
+      throw new FlutterError('Duplicate key found: $nonUniqueKey.\n');
+    return true;
+  });
+  return false;
+}

packages/flutter/lib/src/widgets/framework.dart

@@ -1937,7 +1937,7 @@ class SingleChildRenderObjectElement extends RenderObjectElement {
 /// Instantiation of RenderObjectWidgets that can have a list of children
 class MultiChildRenderObjectElement extends RenderObjectElement {
   MultiChildRenderObjectElement(MultiChildRenderObjectWidget widget) : super(widget) {
-    assert(!debugHasDuplicateKeys(widget, widget.children));
+    assert(!debugChildrenHaveDuplicateKeys(widget, widget.children));
   }
 
   @override

packages/flutter/lib/src/widgets/virtual_viewport.dart

@@ -173,8 +173,9 @@ abstract class VirtualViewportElement extends RenderObjectElement {
       Key key = child.key != null ? new ValueKey<Key>(child.key) : new ValueKey<int>(childIndex);
       newWidgets[i] = new RepaintBoundary(key: key, child: child);
     }
-    assert(!debugHasDuplicateKeys(widget, newWidgets));
-    _materializedChildren = updateChildren(_materializedChildren, newWidgets);
+
+    assert(!debugChildrenHaveDuplicateKeys(widget, newWidgets));
+    _materializedChildren = updateChildren(_materializedChildren, newWidgets.toList());
   }
 
   @override