fix poor performance with firefox when using eval as devtool (facebook#924)

Enable sourcemaps in development

Author: ekaradon

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

@@ -33,11 +33,9 @@ var env = getClientEnvironment(publicUrl);
 // It is focused on developer experience and fast rebuilds.
 // The production configuration is different and lives in a separate file.
 module.exports = {
-  // This makes the bundle appear split into separate modules in the devtools.
-  // We don't use source maps here because they can be confusing:
-  // https://github.com/facebookincubator/create-react-app/issues/343#issuecomment-237241875
-  // You may want 'cheap-module-source-map' instead if you prefer source maps.
-  devtool: 'eval',
+  // You may want 'eval' instead if you prefer to see the compiled output in DevTools.
+  // See the discussion in https://github.com/facebookincubator/create-react-app/issues/343.
+  devtool: 'cheap-module-source-map',
   // These are the "entry points" to our application.
   // This means they will be the "root" imports that are included in JS bundle.
   // The first two entry points enable "hot" CSS and auto-refreshes for JS.