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

backend/node_modules/autoprefixer/lib/at-rule.js

@@ -1,35 +0,0 @@
-let Prefixer = require('./prefixer')
-
-class AtRule extends Prefixer {
-  /**
-   * Clone and add prefixes for at-rule
-   */
-  add(rule, prefix) {
-    let prefixed = prefix + rule.name
-
-    let already = rule.parent.some(
-      i => i.name === prefixed && i.params === rule.params
-    )
-    if (already) {
-      return undefined
-    }
-
-    let cloned = this.clone(rule, { name: prefixed })
-    return rule.parent.insertBefore(rule, cloned)
-  }
-
-  /**
-   * Clone node with prefixes
-   */
-  process(node) {
-    let parent = this.parentPrefix(node)
-
-    for (let prefix of this.prefixes) {
-      if (!parent || parent === prefix) {
-        this.add(node, prefix)
-      }
-    }
-  }
-}
-
-module.exports = AtRule