Update cli description for a couple of benchmarks.

pkg/kernel/test/class_hierarchy_bench.dart

Index: pkg/kernel/test/class_hierarchy_bench.dart
diff --git a/pkg/kernel/test/class_hierarchy_bench.dart b/pkg/kernel/test/class_hierarchy_bench.dart
index 11fdb7a20531b9f9b3640073ef674f1c23b7cd55..5f2f34f9ff18d2de545d2a0ab6e3e68ab3ee86ba 100644
--- a/pkg/kernel/test/class_hierarchy_bench.dart
+++ b/pkg/kernel/test/class_hierarchy_bench.dart
@@ -18,7 +18,7 @@ ArgParser argParser = new ArgParser()
       defaultsTo: '1');
 
 String usage = '''
-Usage: class_hierarchy_bench [options] FILE.dart
+Usage: class_hierarchy_bench [options] FILE.dill
 
 Options:
 ${argParser.usage}