Fix warnings_checker.dart handling of multitests

tests/language_strong/compile_time_constant_p_test.dart

 // Copyright (c) 2012, 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 "package:expect/expect.dart";
 
 class A {
   const A(
-    this.x // /// 01: compile-time error
+    this.x // //# 01: compile-time error
   );
   final x = null;
 }
 
 class B extends A {
   const B();
 }
 
 var b = const B();
 
 main() {
   Expect.equals(null, b.x);
 }