In Sublime Text, I can right click in the text of a file I’m editing that has unsaved changes and “View Unsaved Changes…”. This presents a diff between the saved and unsaved states of the file. Something similar would be extremely handy in Atom as well. I searched, but couldn’t find any packages that do this either.
If you save to a git repo often you can compare the current source to the repo with git-diff-popup (my pkg).
There’s one already https://atom.io/packages/unsaved-changes
Show lines changed from last save action