Fix warnings_checker.dart handling of multitests

tests/language/try_catch_test.dart

Index: tests/language/try_catch_test.dart
diff --git a/tests/language/try_catch_test.dart b/tests/language/try_catch_test.dart
index 0d35d28f834f564b01779bdd5ce30270b36b82fa..2b126e460da5366b142fee561a07c5f9551e2d99 100644
--- a/tests/language/try_catch_test.dart
+++ b/tests/language/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");
     }
   }