I recently pulled in the latest Laravel 5.3. In the gulp.js file, there is the newer ES6 arrow function syntax:
gulp.js
elixir(mix => { mix.sass(
Go to Settings > Languages & Frameworks > JavaScript
Settings > Languages & Frameworks > JavaScript
Then change JavaScript language version to ECMAScript 6
JavaScript language version
ECMAScript 6
MacOs: Menu > PhpStorm> Preferences > Languages & Frameworks > JavaScript and change JavaScript language version to ECMAScript 6
Menu > PhpStorm> Preferences > Languages & Frameworks > JavaScript
JavaScript language version to ECMAScript 6