Fix warnings_checker.dart handling of multitests

tests/language_strong/try_catch_test.dart

Index: tests/language_strong/try_catch_test.dart
diff --git a/tests/language_strong/try_catch_test.dart b/tests/language_strong/try_catch_test.dart
index 4a41dd1efd4e54d94136b8ea7e189469a10dc751..f7d7e97938f4071bdfb461462b079aa1736b5b12 100644
--- a/tests/language_strong/try_catch_test.dart
+++ b/tests/language_strong/try_catch_test.dart
@@ -168,7 +168,7 @@ class TryCatchTest {
     } on String catch (e) {
       // Compile-time constants in unreachable catch blocks are still
       // compiled.
-      const y = x[0]; // /// 01: compile-time error
+      const y = x[0]; // //# 01: compile-time error
       Expect.fail("unreachable");
     }
   }