Fix warnings_checker.dart handling of multitests

tests/language/mixin_invalid_inheritance1_test.dart

Index: tests/language/mixin_invalid_inheritance1_test.dart
diff --git a/tests/language/mixin_invalid_inheritance1_test.dart b/tests/language/mixin_invalid_inheritance1_test.dart
index 2497c86db8855826eb9021e12bc24f0b1941cebd..ab30e92c918337c859ae27a05ea1de113b1b496b 100644
--- a/tests/language/mixin_invalid_inheritance1_test.dart
+++ b/tests/language/mixin_invalid_inheritance1_test.dart
@@ -3,9 +3,9 @@
 // BSD-style license that can be found in the LICENSE file.
 
 class C<T> extends Object
-  with Malformed // /// 01: compile-time error
-  with T // /// 02: compile-time error
-  with T<int> // /// 03: compile-time error
+  with Malformed // //# 01: compile-time error
+  with T // //# 02: compile-time error
+  with T<int> // //# 03: compile-time error
 {}
 
 main() => new C<C>();