Fix external CSS imports (#2680)

* Fix external CSS imports

* Add external CSS import to integration test

* Fix assertion

packages/react-scripts/config/webpack.config.prod.js

@@ -228,6 +228,9 @@ module.exports = {
                 {
                   loader: require.resolve('postcss-loader'),
                   options: {
+                    // Necessary for external CSS imports to work
+                    // https://github.com/facebookincubator/create-react-app/issues/2677
+                    ident: 'postcss',
                     plugins: () => [
                       require('postcss-flexbugs-fixes'),
                       autoprefixer({