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This differs from the corresponding code in Rails apps because it takes no args, so any groups in the Gemfile will be ignored. A comment in the Gemfile that says not to use groups without editing this line would be really helpful.
I realize that to anyone with knowledge of Bundler, debugging this issue would take a matter of minutes, but keep in mind that many Rails developers might have minimal knowledge of Bundler. One of Rails' excellent features is that it allows you to use Bundler without having to get involved in (or have any clue about) the fine details of it. So it took me about 8 hours to figure this out on my own--having never used Bundler.require before--and a large part of the problem is that the error being raised had no search terms that are related to Bundler.
See http://stackoverflow.com/questions/13630456/sass-import-directive-when-used-in-rails-engine-cant-find-assets-in-plugins/13631079#13631079 for the issue I was debugging.
Please submit a pull request with this change, as we don't take feature requests on the issue tracker. thanks!
This should be fixed now. I submitted a PR #8262 to load the groups too. I've backported the change to 3-2-stable so you can give it a shot.