Adds dev writeToDisk option for react-scripts start.

Applies the patch from this abandoned PR:

facebook#7812

Based on this 4-year-old issue about emitting dev builds to disk:

facebook#1070

To use, call `react-scripts start --writeToDisk`. This will emit
dev environment built files to the normally-production output
path.

Author: mildmojo

packages/react-scripts/config/paths.js

@@ -24,7 +24,7 @@ const resolveApp = relativePath => path.resolve(appDirectory, relativePath);
 // We can't use a relative path in HTML because we don't want to load something
 // like /todos/42/static/js/bundle.7289d.js. We have to know the root.
 const publicUrlOrPath = getPublicUrlOrPath(
-  process.env.NODE_ENV === 'development',
+  process.env.NODE_ENV === 'development' && !process.argv.includes('--writeToDisk'),
   require(resolveApp('package.json')).homepage,
   process.env.PUBLIC_URL
 );

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

@@ -68,6 +68,9 @@ module.exports = function(webpackEnv) {
   const isEnvDevelopment = webpackEnv === 'development';
   const isEnvProduction = webpackEnv === 'production';
 
+  // Passing this flag will write build files to the production build path
+  const writeToDisk = process.argv.includes('--writeToDisk');
+
   // Variable used for enabling profiling in Production
   // passed into alias object. Uses a flag if passed into the build command
   const isEnvProductionProfile =
@@ -176,7 +179,7 @@ module.exports = function(webpackEnv) {
     },
     output: {
       // The build folder.
-      path: isEnvProduction ? paths.appBuild : undefined,
+      path: isEnvProduction || writeToDisk ? paths.appBuild : undefined,
       // Add /* filename */ comments to generated require()s in the output.
       pathinfo: isEnvDevelopment,
       // There will be one main bundle, and one file per asynchronous chunk.

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

@@ -134,5 +134,6 @@ module.exports = function(proxy, allowedHost) {
       // https://github.com/facebook/create-react-app/issues/2272#issuecomment-302832432
       app.use(noopServiceWorkerMiddleware(paths.publicUrlOrPath));
     },
+    writeToDisk: process.argv.includes('--writeToDisk'),
   };
 };

packages/react-scripts/scripts/start.js

@@ -155,7 +155,9 @@ checkBrowsers(paths.appPath, isInteractive)
       }
 
       console.log(chalk.cyan('Starting the development server...\n'));
-      openBrowser(urls.localUrlForBrowser);
+      if (!process.argv.includes('--writeToDisk')) {
+        openBrowser(urls.localUrlForBrowser);
+      }
     });
 
     ['SIGINT', 'SIGTERM'].forEach(function(sig) {