Extract ClassHierarchyImpl implementation from ClassHierarchy.

pkg/kernel/test/class_hierarchy_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.
 import 'package:kernel/kernel.dart';
 import 'package:kernel/class_hierarchy.dart';
 import 'package:args/args.dart';
 import 'class_hierarchy_basic.dart';
 import 'dart:io';
 
@@ skipping 25 matching lines @@
   }
   String filename = options.rest.single;
 
   Program program = loadProgramFromBinary(filename);
 
   int copyCount = int.parse(options['count']);
 
   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());
   }
 
   print('$copyCount copies built');
 
   if (args.contains('-v')) {
     // Use of the list for something to avoid premature GC.
     int size = 0;
     for (var classHierarchy in keepAlive) {
       size += classHierarchy.getSuperTypeHashTableSize();
     }
     print(size);
   }
 }