Fix warnings_checker.dart handling of multitests

tests/language/const_map2_test.dart

 // Copyright (c) 2014, 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 factory A() = B;
 }
 
 class B implements A {
   const B();
 
-  operator ==(o) => true; // /// 00: compile-time error
+  operator ==(o) => true; // //# 00: compile-time error
 }
 
 @NoInline() @AssumeDynamic()
 confuse(x) => x;
 
 main() {
   // It is a compile-time error if the key type overrides operator ==.
   var m = const { const A(): 42 };
   Expect.equals(42, m[confuse(const B())]);
 
   m = const { "foo": 99, const A(): 499 };
   Expect.equals(499, m[confuse(const B())]);
 }