Fix prod css/sass sourcemaps (#1793) (#2282)

* Fix prod css/sass sourcemaps (#1793)

Sass sourcemaps fail to generate in production builds due to cssnano
combined with OptimizeCSSAssetsPlugin.  Moving cssnano into
postcss-loader resolves issue.

More detail https://www.webfoobar.com/node/109

Note eval-source-maps still will not work at all for CSS, so
implementers must use `Mix.sourceMaps(true, 'source-map')` to allow
development builds to successfully generate sourcemaps.

* Add cssnano to postcss plugins

* Allow cssNano config options to be specified

* remove comment

src/builder/webpack-default.js

@@ -39,14 +39,7 @@ module.exports = function() {
 
         optimization: Mix.inProduction()
             ? {
-                  minimizer: [
-                      new TerserPlugin(Config.terser),
-                      new OptimizeCSSAssetsPlugin({
-                          cssProcessorPluginOptions: {
-                              preset: ['default', Config.cssNano]
-                          }
-                      })
-                  ]
+                  minimizer: [new TerserPlugin(Config.terser)]
               }
             : {},
 

src/components/Preprocessor.js

@@ -65,6 +65,14 @@ class Preprocessor {
                                     );
                                 }
 
+                                if (Mix.inProduction()) {
+                                    plugins.push(
+                                        require('cssnano')({
+                                            preset: ['default', Config.cssNano]
+                                        })
+                                    );
+                                }
+
                                 return plugins;
                             })()
                         }