Remove ClassHierarchy factory constructor.

pkg/kernel/test/class_hierarchy_bench.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:math';
 import 'dart:io';
@@ skipping 23 matching lines @@
     print('Exactly one file must be given');
     exit(1);
   }
   String filename = options.rest.single;
 
   Program program = loadProgramFromBinary(filename);
 
   ClassHierarchy buildHierarchy() {
     return options['basic']
         ? new BasicClassHierarchy(program)
-        : new ClassHierarchy(program);
+        : new ClosedWorldClassHierarchy(program);
   }
 
   var watch = new Stopwatch()..start();
   buildHierarchy();
   int coldBuildTime = watch.elapsedMilliseconds;
   watch.reset();
   const int numBuildTrials = 100;
   for (int i = 0; i < numBuildTrials; i++) {
     buildHierarchy();
   }
@@ skipping 281 matching lines @@
       : list[mid].toDouble();
 }
 
 num sum(Iterable<num> values) {
   num result = 0;
   for (var x in values) {
     result += x;
   }
   return result;
 }