Fix hanging start

CRA 3 opts-in to a new webpack 5 feature early to save memory: `futureEmitAssets`
facebook#6696
This optimization only stores the file sizes during webpack compilation, no longer storing the original source
webpack/webpack#8642
This is incompatible with our webpack-dev-server customization setting of `writeToDisk`
webpack/webpack#8642 (comment)
which tries to read the source after emit, which throws an exception, but that error is swallowed and the `npm start` just hangs.
Before we update to webpack 5, where this feature will be the default, we need to stop writing to disk, or find another way to do it than the `writeToDisk` flag.

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

@@ -173,7 +173,8 @@ module.exports = function(webpackEnv) {
         ? 'static/js/[name].[contenthash:8].js'
         : isEnvDevelopment && 'static/js/bundle.js',
       // TODO: remove this when upgrading to webpack 5
-      futureEmitAssets: true,
+      // FS - writeToDisk and futureEmitAssets are not compatible, so defaulting back to false: https://github.com/webpack/webpack/pull/8642#issuecomment-455342804
+      futureEmitAssets: false,
       // There are also additional JS chunk files if you use code splitting.
       chunkFilename: isEnvProduction
         ? 'static/js/[name].[contenthash:8].chunk.js'