Fix warnings_checker.dart handling of multitests

tests/language/illegal_declaration_test.dart

Index: tests/language/illegal_declaration_test.dart
diff --git a/tests/language/illegal_declaration_test.dart b/tests/language/illegal_declaration_test.dart
index 25411518c3aa1d3baecff4086c8f741437b520cd..1b935f98ca7bff06df8e4e35c44a61e4cc916177 100644
--- a/tests/language/illegal_declaration_test.dart
+++ b/tests/language/illegal_declaration_test.dart
@@ -2,6 +2,6 @@
 // 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.
 
-[ // /// 01: compile-time error
+[ // //# 01: compile-time error
 
 main() {}