I tried to focus on it and View Menu -> Close Pane but its still there. I had it hidden before.
You can toggle the Tree View with the “Tree View: Toggle” command, Cmd+\ on OS X or Ctrl+\ on other platforms.
Thank you, you would think that would be in the Panels menu.
Another one is, which works on a German keyboard:
And for my swedish mac:
It works for me just like in sublime
CTRL+K+B for toggle (Ubuntu)
Works on my mac
On latin american keyboard also works
You can find the TreeView-menu under
Packages > Tree View in the top level menu.
Apparently installing Nuclide disables Tree View. Maybe. Somehow it got disabled after I installed Nuclide. You have to re-enable it to get the files pane back.