Fix warnings_checker.dart handling of multitests

tests/language_strong/deferred_duplicate_prefix3_test.dart

Index: tests/language_strong/deferred_duplicate_prefix3_test.dart
diff --git a/tests/language_strong/deferred_duplicate_prefix3_test.dart b/tests/language_strong/deferred_duplicate_prefix3_test.dart
index dc3e85bc9ff3b0ff8996d4c48f25c84307876c0f..08d8bd19ec574d704dc56ff33c8ce1d31378472d 100644
--- a/tests/language_strong/deferred_duplicate_prefix3_test.dart
+++ b/tests/language_strong/deferred_duplicate_prefix3_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_lib2.dart" deferred as lib; /// 01: continued
+import "deferred_prefix_constraints_lib.dart" deferred as lib; //# 01: compile-time error
+import "deferred_prefix_constraints_lib2.dart" deferred as lib; //# 01: continued
 
 void main() {}