Remove ClassHierarchy factory constructor.

pkg/kernel/test/treeshaker_membench.dart

Index: pkg/kernel/test/treeshaker_membench.dart
diff --git a/pkg/kernel/test/treeshaker_membench.dart b/pkg/kernel/test/treeshaker_membench.dart
index 596ef87b9165b5ab1ca4488ebf3af6f4f75b6770..ee9e0874a67c810fcae8ffeb0166c7131d55af42 100644
--- a/pkg/kernel/test/treeshaker_membench.dart
+++ b/pkg/kernel/test/treeshaker_membench.dart
@@ -39,7 +39,7 @@ main(List<String> args) {
   bool strongMode = options['strong'];
 
   Program program = loadProgramFromBinary(filename);
-  ClassHierarchy hierarchy = new ClassHierarchy(program);
+  ClassHierarchy hierarchy = new ClosedWorldClassHierarchy(program);
   CoreTypes coreTypes = new CoreTypes(program);
 
   int copyCount = int.parse(options['count']);