Add proper API for creating private symbols wrt a library.

tests/lib/mirrors/intercepted_object_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.
 
 // Ensure that objects handled specially by dart2js can be reflected on.
 
-library test.intercepted_class_test;
+library test.intercepted_object_test;
 
 import 'dart:mirrors';
 
 import 'stringify.dart' show stringify, expect;
 
 import 'intercepted_class_test.dart' show checkClassMirror;
 
 checkObject(object, String name) {
   checkClassMirror(reflect(object).type, name);
 }
 
 main() {
   checkObject('', 'String');
   checkObject(1, 'int');
   checkObject(1.5, 'double');
   checkObject(true, 'bool');
   checkObject(false, 'bool');
   checkObject([], 'List');
 }