Remove ClassHierarchy factory constructor.

pkg/kernel/test/class_hierarchy_bench.dart

Index: pkg/kernel/test/class_hierarchy_bench.dart
diff --git a/pkg/kernel/test/class_hierarchy_bench.dart b/pkg/kernel/test/class_hierarchy_bench.dart
index 02d6b0cc93150004aaa917a1441ea1f0f5dfc434..03060abf50e61f76dc41cc8e9a2150fbbd705a8f 100644
--- a/pkg/kernel/test/class_hierarchy_bench.dart
+++ b/pkg/kernel/test/class_hierarchy_bench.dart
@@ -41,7 +41,7 @@ main(List<String> args) {
   ClassHierarchy buildHierarchy() {
     return options['basic']
         ? new BasicClassHierarchy(program)
-        : new ClassHierarchy(program);
+        : new ClosedWorldClassHierarchy(program);
   }
 
   var watch = new Stopwatch()..start();