"slash trick" Accessing arbitrary paths in open dialogs in Sierra
Pressing Command-Shift-G in a standard open (or save) dialog will open a "Go to the folder" dialog that accepts unix-style paths (and does tab-completion). If you want to open a hidden file, pressing Command-Shift-Period in the open/save dialog will show hidden items.

(BTW, the Command-Shift-G shortcut also works in the Finder, and is equivalent to choosing Go menu > Go to Folder. Unfortunately, Command-Shift-Period doesn't work in Finder, only in open/save dialogs.)