Fix warnings_checker.dart handling of multitests

tests/language/mixin_supertype_subclass2_test.dart

Index: tests/language/mixin_supertype_subclass2_test.dart
diff --git a/tests/language/mixin_supertype_subclass2_test.dart b/tests/language/mixin_supertype_subclass2_test.dart
index d7ee5629117111cbf5be84ef85996ad79e08fe6b..d1b7f9149eed510b72ef0e1c3743f959c26054c4 100644
--- a/tests/language/mixin_supertype_subclass2_test.dart
+++ b/tests/language/mixin_supertype_subclass2_test.dart
@@ -18,11 +18,11 @@ class E extends B with C implements D {}
 class F extends E {}
 class A extends E with M {}
 class M
-  extends B /// 01: ok
-  extends C /// 02: static type warning
-  extends D /// 03: ok
-  extends E /// 04: ok
-  extends F /// 05: static type warning
+  extends B //# 01: ok
+  extends C //# 02: static type warning
+  extends D //# 03: ok
+  extends E //# 04: ok
+  extends F //# 05: static type warning
   {}
 
 main() {