Update cli description for a couple of benchmarks.

pkg/kernel/test/class_hierarchy_membench.dart

Index: pkg/kernel/test/class_hierarchy_membench.dart
diff --git a/pkg/kernel/test/class_hierarchy_membench.dart b/pkg/kernel/test/class_hierarchy_membench.dart
index 3c73bc5d7e5c98deb74944bfecb905a5fedc8e45..d8b05ec8f334c4d3b8d5e7d73a4e6aa3abeb13c7 100644
--- a/pkg/kernel/test/class_hierarchy_membench.dart
+++ b/pkg/kernel/test/class_hierarchy_membench.dart
@@ -16,7 +16,7 @@ ArgParser argParser = new ArgParser()
       defaultsTo: '300');
 
 String usage = """
-Usage: class_hierarchy_membench [options] FILE.dart
+Usage: class_hierarchy_membench [options] FILE.dill
 
 Options:
 ${argParser.usage}