manually running postcss and adding it to vendor

ember-cli-build.js

@@ -2,22 +2,9 @@
 
 const EmberAddon = require('ember-cli/lib/broccoli/ember-addon');
 
-const PresetEnv = require('postcss-preset-env');
 
 module.exports = function(defaults) {
   let app = new EmberAddon(defaults, {
-    postcssOptions: {
-      compile: {
-        enabled: true,
-        plugins: [
-          {
-            module: PresetEnv,
-            options: { stage: 3 },
-            features: { 'nesting-rules': true }
-          }
-        ]
-      }
-    },
     minifyCSS: {
       enabled: false
     },

index.js

@@ -1,25 +1,35 @@
 'use strict';
 
+const CssImport = require('postcss-import');
 const PresetEnv = require('postcss-preset-env');
-
+var broccoliPostCSS = require('broccoli-postcss')
+const mergeTrees = require('broccoli-merge-trees');
+var get = require('lodash.get');
+const { join } = require('path');
 
 module.exports = {
   name: require('./package').name,
 
-  options: {
-    postcssOptions: {
-      compile: {
-        enabled: true,
-        plugins: [
-          {
-            module: PresetEnv,
-            options: { stage: 3 },
-            features: {
-              'nesting-rules': true
-            }
-          }
-        ]
-      }
-    }
+  treeForAddon() {
+    var tree = this._super(...arguments);
+
+    // I don't know exactly why targets is private so I am using `get()` to make
+    // sure that it isn't missing
+    let overrideBrowserslist = get(this, 'app.project._targets.browsers');
+
+    let newFile = broccoliPostCSS(join(__dirname, 'prebuilt-styles'), {
+      plugins: [
+        CssImport({
+          path: join(__dirname, 'prebuilt-styles'),
+          addModulesDirectories: [__dirname]
+        }),
+        PresetEnv({
+          stage: 3,
+          features: { 'nesting-rules': true },
+          overrideBrowserslist,
+        })
+      ]});
+
+    return mergeTrees([tree, newFile]);
   },
 };