Add sass and autoprefixer to gulp in visual studio 2015

In Visual studio 2015 (Update 2) a new ASP.NET Core Web Application contains a gulpfile that adds css and js minification. Here is how to add scss and autoprefixing support to that gulpfile.

After adding a new project rename site.css to site.scss

Next install the the required npm packages:

npm install gulp-autoprefixer
npm install gulp-sass

You need to ensure you run these commands from the project folder (not just anywhere) eg for me that means opening a command prompt at

 D:\aaa_development\spikes\GulpSpike\src\GulpSpike

Next, attempt to reference these from gulpfile.js by adding the following lines after the other existing require definitions

var autoprefixer = require('gulp-autoprefixer')
var sass = require('gulp-sass');