Remove ClassHierarchy factory constructor.

pkg/kernel/test/treeshaker_membench.dart

 #!/usr/bin/env dart
 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 library kernel.treeshaker_membench;
 
 import 'package:kernel/kernel.dart';
 import 'package:kernel/transformations/treeshaker.dart';
 import 'package:kernel/class_hierarchy.dart';
 import 'package:kernel/core_types.dart';
@@ skipping 21 matching lines @@
   }
   ArgResults options = argParser.parse(args);
   if (options.rest.length != 1) {
     print('Exactly one file should be given');
     exit(1);
   }
   String filename = options.rest.single;
   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']);
 
   TreeShaker buildTreeShaker() {
     return new TreeShaker(coreTypes, hierarchy, program,
         strongMode: strongMode);
   }
 
   List<TreeShaker> keepAlive = <TreeShaker>[];
   for (int i = 0; i < copyCount; ++i) {
     keepAlive.add(buildTreeShaker());
   }
 
   print('$copyCount copies built');
 
   if (args.contains('-v')) {
     // Use of the list for something to avoid premature GC.
     for (var treeShaker in keepAlive) {
       treeShaker.getClassRetention(coreTypes.objectClass);
     }
   }
 }