Fix warnings_checker.dart handling of multitests

tests/lib/async/future_or_only_in_async_test.dart

 // Copyright (c) 2016, 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.
 
 // `FutureOr<T>` is only visible when `dart:async` is imported.
 
 dynamic foo(dynamic x) {
   return x as
-      FutureOr< // /// 00: runtime error, static type warning
+      FutureOr< // //# 00: runtime error, static type warning
         int
-      > //         /// 00: continued
+      > //         //# 00: continued
   ;
 }
 
 main() {
   if (499 != foo(499)) throw "bad";
 }