Fix warnings_checker.dart handling of multitests

tests/compiler/dart2js_extra/regress/4562_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.
 
 void main() {
   var foo = new Foo();
-  var bar = new Bar(); /// 01: compile-time error
+  var bar = new Bar(); //# 01: compile-time error
 }
 
 class Foo {
   factory Foo() => null;
 }
 
 class Bar extends Foo {
-  Bar(); /// 01: continued
+  Bar(); //# 01: continued
 }