This means the js file is blocked on modern browsers by default if you are using SSL admin because it tries to load the file over http instead of https.
The proper way to do this is to use
plugins_url(), as noted here: http://codex.wordpress.org/Determining_Plugin_and_Content_Directories
WordPress makes use of the following constants when determining the path to the content and plugin directories. These should not be used directly by plugins or themes
I've patched this on my local version, any chance you can add this to your next update?