add LibraryIdentifier

test/bar.dart

 // Copyright (c) 2015, 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.
 @initializeTracker
 library initialize.test.bar;
 
 import 'foo.dart';
-import 'initialize_tracker.dart';
+import 'package:initialize/src/initialize_tracker.dart';
 
 // Foo should be initialized first.
 @initializeTracker
 class Bar extends Foo {}
 
 @initializeTracker
 bar() {}