Update cli description for a couple of benchmarks.

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';
 
 ArgParser argParser = new ArgParser()
   ..addFlag('basic', help: 'Measure the basic implementation', negatable: false)
   ..addOption('count',
       abbr: 'c',
       help: 'Build N copies of the class hierarchy',
       defaultsTo: '300');
 
 String usage = """
-Usage: class_hierarchy_membench [options] FILE.dart
+Usage: class_hierarchy_membench [options] FILE.dill
 
 Options:
 ${argParser.usage}
 """;
 
 /// Builds N copies of the class hierarchy for the given program.
 /// Pass --print-metrics to the Dart VM to measure the memory use.
 main(List<String> args) {
   if (args.length == 0) {
     print(usage);
@@ skipping 25 matching lines @@
 
   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);
   }
 }