build.dart ignores part 'xxx.dart' files from an imported components source files

[DartBot]

This issue was originally filed by @zoechi

What steps will reproduce the problem?
1.
I have a library my_component containing a Polymer component.
Source files are located in lib
The Dart source consists of my_comp.html, my_comp.dart, helper_class_a.dart to helper_class_g.dart

my_comp.dart starts with
  library my_component;
  (imports go here)
  part 'helper_class_a.dart';
  ...
  part 'helper_class_g.dart';

helper_class_(n).dart start with
  part of my_component;
2.
Import the component and use it in a web application project

run the build script
run dart2js

3.
Part of the output:
Copying package:browser/dart.js → deploy/out/packages/browser/dart.js
Copying package:browser/interop.js → deploy/out/packages/browser/interop.js
Compiling web/out/index.html_bootstrap.dart → deploy/out/index.html_bootstrap.dart.js
web/out/_from_packages/my_component/my_component.dart:57:1: Error: Can't read 'file:///home/myuser/dart/demoapp/web/out/_from_packages/my_component/helper_class_a.dart' (Error reading 'web/out/_from_packages/my_component/data.dart' (OS Error: No such file or directory, errno = 2)).
part 'helper_class_a.dart';
^^^^^^^^^^^^^^^^^^^^^^^^^^^

What is the expected output? What do you see instead?
dart2js should process the files so that the output is a valid web app.

What version of the product are you using? On what operating system?
Dart 0.6.21
Debian Linux x64 jessie

Please provide any additional information below.
I have a class (Model) from helper_class_c.dart referenced in main().
helper_class_c.dart is the only file of all helper_class_n.dart files that was copied to web/out/_from_packages/my_component (beside the my_component.dart file)

build.dart:
import 'dart:io';
import 'package:polymer/component_build.dart';
main() {
  var pages = ['web/index.html'];
  var args = new Options().arguments;
  args.addAll(['--', '--deploy']);
  build(args, pages);
}

[sigmundch]

thanks for reporting this bug! I'm currently doing a bunch of work that will replace and fix a lot of issues in our deploy step. This should be handled now by the new system but we should double check when we switch over.

We have a couple related issues:

• Issue rewrite compiler as barback transformers #12511 tracks most of the work items building the new deploy system.
• Issue use barback transformers as our new deploy step #12814 tracks switching over to the new deploy system. I'll mark this bug as a duplicate of that one.

  ---

  Set owner to @sigmundch.
  Added Area-Polymer, Duplicate labels.
  Marked as being merged into use barback transformers as our new deploy step #12814.