Update cli description for a couple of benchmarks.

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';
 
 ArgParser argParser = new ArgParser()
   ..addFlag('basic', help: 'Measure the basic implementation', negatable: false)
   ..addOption('cycle',
       abbr: 'c',
       help: 'Build N copies of the class hierarchy and cycle queries '
           'between them',
       defaultsTo: '1');
 
 String usage = '''
-Usage: class_hierarchy_bench [options] FILE.dart
+Usage: class_hierarchy_bench [options] FILE.dill
 
 Options:
 ${argParser.usage}
 ''';
 
 main(List<String> args) {
   if (args.length == 0) {
     print(usage);
     exit(1);
   }
@@ skipping 304 matching lines @@
       : list[mid].toDouble();
 }
 
 num sum(Iterable<num> values) {
   num result = 0;
   for (var x in values) {
     result += x;
   }
   return result;
 }