webpack: use source maps for production build

webpack/webpack.prod.conf.js

@@ -3,6 +3,7 @@ const NgAnnotatePlugin = require('ng-annotate-webpack-plugin');
 
 var conf = baseWebpackConfig;
 conf.mode = 'production';
+conf.devtool = 'source-map';
 conf.plugins.push(new NgAnnotatePlugin());
 
 module.exports = baseWebpackConfig;