change to source-order crawling of directives instead of alphabetical

test/parts/foo.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.
+part of initialize.initializer_parts_test;
+
 @initializeTracker
-library test_package.bar;
+class Foo {}
 
-import 'package:initialize/src/initialize_tracker.dart';
+@initializeTracker
+foo() {}