Fix warnings_checker.dart handling of multitests

tests/language/null_bottom_test.dart

Index: tests/language/null_bottom_test.dart
diff --git a/tests/language/null_bottom_test.dart b/tests/language/null_bottom_test.dart
index 029c03190d75c123377a1cfe01fa242863c9a428..e13ad67f5a9719598ea5ec35ba85c860679c23af 100644
--- a/tests/language/null_bottom_test.dart
+++ b/tests/language/null_bottom_test.dart
@@ -113,7 +113,7 @@ void testFunctionTypes() {
     // If type promotion succeeds, the static type is int->Null, otherwise
     // it's Null->int.
     fun2(42);  // Should not give a warning after type promotion.
-    fun2(null).abs(); // /// 03: runtime error
+    fun2(null).abs(); // //# 03: runtime error
   }
 }