Fix warnings_checker.dart handling of multitests

tests/corelib/hidden_library2_test.dart

Index: tests/corelib/hidden_library2_test.dart
diff --git a/tests/corelib/hidden_library2_test.dart b/tests/corelib/hidden_library2_test.dart
index dd12cd28f151e679dc8db2da7078b2bb0e186a30..637cd940779b85b9f1f6a5ceec8f615cdc1811f8 100644
--- a/tests/corelib/hidden_library2_test.dart
+++ b/tests/corelib/hidden_library2_test.dart
@@ -9,7 +9,7 @@ main() {
   print(['x'].where((_) {
     // We actually don't really care for the successful case. We just want to
     // make sure that the test doesn't crash when it is negative.
-    throw 'fisk'; // /// 01: runtime error
+    throw 'fisk'; // //# 01: runtime error
     return true;
   }).toList());
 }