It appears that the repository has undergone several changes and renamings:

backend/node_modules/postcss-js/async.js

@@ -1,15 +0,0 @@
-let postcss = require('postcss')
-
-let processResult = require('./process-result')
-let parse = require('./parser')
-
-module.exports = function async(plugins) {
-  let processor = postcss(plugins)
-  return async input => {
-    let result = await processor.process(input, {
-      parser: parse,
-      from: undefined
-    })
-    return processResult(result)
-  }
-}