Fix warnings_checker.dart handling of multitests

tests/language/const_constructor_nonconst_field_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 {
-  final int i = f(); /// 01: compile-time error
+  final int i = f(); //# 01: compile-time error
   final int j = 1;
   const A();
 }
 int f() {
   return 3;
 }
 main() {
   Expect.equals(const A().j, 1);
 }