On macOS 10.12 Sierra, flushing DNS uses the same commands as 10.11 and previous.
To flush your DNS on macOS 10.12 Sierra, issue this command in terminal:
sudo dscacheutil -flushcache;sudo killall -HUP mDNSResponder;say flushed
At the prompt, enter your password and hit the enter key. When it's done flushing your DNS, it will say so.