Fix warnings_checker.dart handling of multitests

tests/language_strong/generic_methods_simple_as_expression_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 that type parameters in generic methods can be used in as-expressions.
 
 library generic_methods_simple_as_expression_test;
 
 import "package:expect/expect.dart";
 
 T cast<T>(dynamic obj) {
   return obj as T;
 }
 
 main() {
-  Expect.equals(cast<num>(42), 42); /// 01: ok
-  cast<String>(42); /// 02: runtime error
+  Expect.equals(cast<num>(42), 42); //# 01: ok
+  cast<String>(42); //# 02: runtime error
 }