Fix warnings_checker.dart handling of multitests

tests/language/duplicate_interface_negative_test.dart

 // Copyright (c) 2013, 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.
 // Check fail because of cycles in super class relationship.
 
 library duplicateInterfaceNegativeTest;
 
 import "duplicate_interface_lib.dart" as alib;
 import "duplicate_interface_lib.dart" show InterfA;
 
 
 // Expect error since InterfA and alib.InterfA refer to the same interface.
-class Foo implements InterfA, alib.InterfA { } /// compile-time error
+class Foo implements InterfA, alib.InterfA { } //# compile-time error
 
 
 main() {
   Expect.isTrue(new Foo() is InterfA);
   Expect.isTrue(new Foo() is alib.InterfA);
 }