Fix warnings_checker.dart handling of multitests

tests/language_strong/deferred_duplicate_prefix2_test.dart

Index: tests/language_strong/deferred_duplicate_prefix2_test.dart
diff --git a/tests/language_strong/deferred_duplicate_prefix2_test.dart b/tests/language_strong/deferred_duplicate_prefix2_test.dart
index 0be28851ac020f62a89fd0e108557f95956d1ec0..1edfeca7b18b08ececc8c2cde0b737b93540dffc 100644
--- a/tests/language_strong/deferred_duplicate_prefix2_test.dart
+++ b/tests/language_strong/deferred_duplicate_prefix2_test.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import "deferred_prefix_constraints_lib.dart" deferred as lib; // /// 01: compile-time error
+import "deferred_prefix_constraints_lib.dart" deferred as lib; // //# 01: compile-time error
 import "deferred_prefix_constraints_lib2.dart" as lib;
 
 void main() {}