Enable futureEmitAssets in webpack config (#6696)

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

@@ -163,6 +163,8 @@ module.exports = function(webpackEnv) {
       filename: isEnvProduction
         ? 'static/js/[name].[contenthash:8].js'
         : isEnvDevelopment && 'static/js/bundle.js',
+      // TODO: remove this when upgrading to webpack 5
+      futureEmitAssets: true,
       // There are also additional JS chunk files if you use code splitting.
       chunkFilename: isEnvProduction
         ? 'static/js/[name].[contenthash:8].chunk.js'