Fix warnings_checker.dart handling of multitests

tests/language_strong/regress_22936_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.
 
 // Regression test for issue 22936.
 
 import 'package:expect/expect.dart';
 
 bool fooCalled = false;
 
 foo() {
   fooCalled = true;
   return null;
 }
 
 main() {
   final x = null;
   try {
-    x = //   /// 01: static type warning
+    x = //   //# 01: static type warning
         foo();
   } on NoSuchMethodError {
   }
   Expect.isTrue(fooCalled);
 }