Fix warnings_checker.dart handling of multitests

tests/language/regress_28217_test.dart

 // Copyright (c) 2017, 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.
 
 // Test non-existing redirecting constructor and
 // redirecting to a factory constructor.
 
 class B {
-  B() : this.a(); //   /// none: compile-time error
-  factory B.a() { } // /// 01: compile-time error
-  B.a(); //            /// 02: ok
+  B() : this.a(); //   //# none: compile-time error
+  factory B.a() { } // //# 01: compile-time error
+  B.a(); //            //# 02: ok
 }
 
 main() => new B();