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

backend/node_modules/postcss/lib/document.js

@@ -1,33 +0,0 @@
-'use strict'
-
-let Container = require('./container')
-
-let LazyResult, Processor
-
-class Document extends Container {
-  constructor(defaults) {
-    // type needs to be passed to super, otherwise child roots won't be normalized correctly
-    super({ type: 'document', ...defaults })
-
-    if (!this.nodes) {
-      this.nodes = []
-    }
-  }
-
-  toResult(opts = {}) {
-    let lazy = new LazyResult(new Processor(), this, opts)
-
-    return lazy.stringify()
-  }
-}
-
-Document.registerLazyResult = dependant => {
-  LazyResult = dependant
-}
-
-Document.registerProcessor = dependant => {
-  Processor = dependant
-}
-
-module.exports = Document
-Document.default = Document