Implement menu links as plugins, including static admin links and views, and custom links with menu_link_content entity, all managed via menu_ui module

This PR is just for interactive review since it's such a big changeset  - please summarize comments at: https://drupal.org/node/2256521
I'm not sure i understand your comment.

There are other methods that build the tree, this just turns it into a render array.  

This method is called from SystemMenuBlock and toolbar.module, so the input data is variable.

This is in any case just copied from elsewhere in HEAD.