Extract ClassHierarchyImpl implementation from ClassHierarchy.

pkg/kernel/test/class_hierarchy_membench.dart

Index: pkg/kernel/test/class_hierarchy_membench.dart
diff --git a/pkg/kernel/test/class_hierarchy_membench.dart b/pkg/kernel/test/class_hierarchy_membench.dart
index d8b05ec8f334c4d3b8d5e7d73a4e6aa3abeb13c7..4909616c2c50870ef72b4919caac0be3efb90225 100644
--- a/pkg/kernel/test/class_hierarchy_membench.dart
+++ b/pkg/kernel/test/class_hierarchy_membench.dart
@@ -43,10 +43,10 @@ main(List<String> args) {
   ClassHierarchy buildHierarchy() {
     return options['basic']
         ? new BasicClassHierarchy(program)
-        : new ClassHierarchy(program);
+        : new ClosedWorldClassHierarchy(program);
   }
 
-  List<ClassHierarchy> keepAlive = <ClassHierarchy>[];
+  List<ClosedWorldClassHierarchy> keepAlive = <ClosedWorldClassHierarchy>[];
   for (int i = 0; i < copyCount; ++i) {
     keepAlive.add(buildHierarchy());
   }