Issue #21438 has been merged into this issue.

Hi, I haven't tried the example directly, but it sounds like the problem here is the path you have to import a shared component. HTML imports are picky and will only recognize that two imports are loading the same file if the imports resolve to the same URL.

Because HTML imports know nothing about Dart, they don't know that a url to "../lib" or a URL to "packages/package_name/" are the same. So you can easily run into a problem with imports that use the form "../lib". That's why we encourage that you use "packages/" urls instead. This is similar to how in Dart we use "package:foo/x.dart" and not "../lib/x.dart", so that everyone uses the same URL to reach the library.

You can find more details on what we do/recommend in this article: https://www.dartlang.org/polymer/app-directories.html

 
In your example, replace both urls, instead of using "../lib":

<link href = '../lib/component/...' rel = 'import'>

use "packages/polymer_prototype_bug" (the name of your package):

<link href = 'packages/polymer_prototype_bug/component/...' rel = 'import'>

Cheers!

Added AsDesigned label.

This comment was originally written by dart.dev.c...@gmail.com

Hi sigmund,
Thanks for the reply.

    <link href = 'packages/polymer_prototype_bug/component/description_form.html' rel = 'import'>
    <link href = 'packages/polymer_prototype_bug/component/unit_form.html' rel = 'import'>

I made the changes you suggested but the result is the same - Exception: Already registered (Polymer) prototype for element h-select-item (package:polymer/src/instance.dart:170)

Hey dart.dev.clarke - I was finally able to try your example, and I can see that the problem is triggered from the html_components package.

It seems that 'package:html_components/input/select_checkbox_menu.dart' has an export of 'package:html_components/input/select_item.dart', which makes us incorrectly register h-select-item twice (the first time when when we discover it from select_item.html, and the second time when we see that export).

Technically it is not incorrect to have that export, so we need to add some more logic to the polymer transformers to handle this special case. If you are the owner of that package, though, and would like to continue making progress without waiting for a fix, then changing the code to remove the export will fix the issue (I just tried that locally and it seemed to work).

Removed Pkg-Polymer label.
Added Pkg-PolymerBuild, Triaged labels.

This comment was originally written by dart.dev.c...@gmail.com

Thanks Sigmund.
I appreciate the help. Yes, I did see the import/export but did not know if it was significant. I am not the author of html_component but I am in contact with the same about this issue. I agree that the polymer transformer logic is necessary to handle the spacial case since the html_component package is 'technically' correct.

About when will a fix be available from the polymer side?

Cheers

Honestly I was hoping I could look this week but I haven't had a chance to do so yet. I'll keep you posted.

Meanwhile there are two hacky tricks you can do to work around the problem:

 - you can download the html_component package, modify it locally and depend on your local copy using a path dependency or dependency_override.

 - you can add a transformer that runs after polymer and remove one line from the generated code. If you run pub build --mode=debug, you will be able to see the output bootstrap.dart file and read what we generate. The generated code will have two lines that register select-item, so your transformer would have to remove one of them.

Cheers

This comment was originally written by dart.dev.clark...@gmail.com

Hello Sigmund,
I hope you will be able to correct the bug this week. I tried what you said but it did not work for me. Could you please zip and send your hack for me to dart.dev.clarke @­ gmail dot com address. At least this will allow me to continue working in the meantime.

Cheers

I just sent a CL out for review: https://codereview.chromium.org/700733002/

Added Started label.

fix submitted, I am assuming this is fixed now

Set owner to @sigmundch.
Added Fixed label.

Yes :) - The fixed package is now available in version 0.15.1+4 from pub.

This comment was originally written by dart.dev.clark...@gmail.com

The fixed package is now available in version 0.15.1+4 from pub.
However, it seems that the problem has resurfaced in polymer 0.15.1+5. Could check it out please.

Thanks

The fix is also there in 0.15.1+5. I verified your example above, and seems to be working here with 0.15.1+5.

I noticed that pub-serve may be caching results of building code in the .pub folder under your project root. It worked for me after I deleted the .pub/ folder under the root of the package. Maybe that's the issue you are seeing?

This issue has been moved to dart-archive/polymer-dart#450.