Fix warnings_checker.dart handling of multitests

tests/language_strong/mixin_invalid_inheritance2_test.dart

Index: tests/language_strong/mixin_invalid_inheritance2_test.dart
diff --git a/tests/language_strong/mixin_invalid_inheritance2_test.dart b/tests/language_strong/mixin_invalid_inheritance2_test.dart
index fa1380a9b79bd499dcf0f47ad56334b3ccd356a2..9edca961595642e0296ef03b909cbfc28eb928c9 100644
--- a/tests/language_strong/mixin_invalid_inheritance2_test.dart
+++ b/tests/language_strong/mixin_invalid_inheritance2_test.dart
@@ -2,12 +2,12 @@
 // 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.
 
-class C<T> = Object with Malformed; // /// 01: compile-time error
-class C<T> = Object with T; // /// 02: compile-time error
-class C<T> = OBject with T<int>; // /// 03: compile-time error
+class C<T> = Object with Malformed; // //# 01: compile-time error
+class C<T> = Object with T; // //# 02: compile-time error
+class C<T> = OBject with T<int>; // //# 03: compile-time error
 
 main() {
-  new C<C>(); // /// 01: continued
-  new C<C>(); // /// 02: continued
-  new C<C>(); // /// 03: continued
+  new C<C>(); // //# 01: continued
+  new C<C>(); // //# 02: continued
+  new C<C>(); // //# 03: continued
 }