Fix warnings_checker.dart handling of multitests

tests/language/regress_23038_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 class C<T> {
   const
-    factory /// 01: compile-time error
+    factory //# 01: compile-time error
     C()
-    = C<C<T>> /// 01: continued
+    = C<C<T>> //# 01: continued
     ;
 }
 
 main() {
   const C<int>();
 }