Fix warnings_checker.dart handling of multitests

tests/lib/mirrors/mirrors_nsm_test.dart

 // Copyright (c) 2011, 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.
 
 library MirrorsTest;
 
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 bool isNSMContainingFieldName(e, String fieldName, bool isSetter) {
@@ skipping 98 matching lines @@
   expectMatchingErrors(() => instanceMirror.getField(#foo),
                        () => new A().foo);
   expectMatchingErrors(() => instanceMirror.setField(#foo, null),
                        () => new A().foo= null);
   expectMatchingErrors(() => instanceMirror.setField(#finalInstance, null),
                        () => new A().finalInstance= null);
 }
 
 main() {
   testMessageContents();
-  testMatchingMessages(); /// dart2js: ok
+  testMatchingMessages(); //# dart2js: ok
 }