Test closed world model after deserialization.

pkg/compiler/lib/src/universe/class_set.dart

Index: pkg/compiler/lib/src/universe/class_set.dart
diff --git a/pkg/compiler/lib/src/universe/class_set.dart b/pkg/compiler/lib/src/universe/class_set.dart
index 818a6d7d3cb0cf3a9f6809b20a161bda2736891b..bed1d6065a78f0d64acf2b4a55f34764648e497c 100644
--- a/pkg/compiler/lib/src/universe/class_set.dart
+++ b/pkg/compiler/lib/src/universe/class_set.dart
@@ -181,6 +181,8 @@ class ClassHierarchyNode {
     _directSubclasses = _directSubclasses.prepend(subclass);
   }
 
+  Iterable<ClassHierarchyNode> get directSubclasses => _directSubclasses;
+
   /// Returns `true` if [other] is contained in the subtree of this node.
   ///
   /// This means that [other] is a subclass of [cls].