Fix warnings_checker.dart handling of multitests

tests/language_strong/regress_23500_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.
 
 import "dart:async";
 import "package:expect/expect.dart";
 
 foo() async {
   try {
     try {
-      await for (var c in new Stream.fromIterable([])) {} // /// 01: ok
-      await 0; // /// 02: ok
+      await for (var c in new Stream.fromIterable([])) {} // //# 01: ok
+      await 0; // //# 02: ok
     } catch (error) {
     }
   } catch (error) {
   }
   throw "error";
 }
 
 main() async {
   var error = "no error";
   try {
     await foo();
   } catch (e) {
     error = e;
   }
   Expect.equals("error", error);
 }