SailsJS is just a framework written via ES5 syntax and it doesn't need to support ES6.
You can write project with ES6 syntax as you usually do and use Babel\Traceur\whatever for running.
My flow is following:
babel
as devDependencies
;npm start
script in package.json
with "start": "babel-node app.js"
;That's it. I can write ES6\7 code in my controllers\models\etc and run the server via npm start
command. It works as usual as you wrote it with ES5 syntax.