Switch back to cheap-module-source-map (facebook#5059)

Author: jasonLaster

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

@@ -77,7 +77,7 @@ module.exports = {
   mode: 'development',
   // You may want 'eval' instead if you prefer to see the compiled output in DevTools.
   // See the discussion in https://github.com/facebook/create-react-app/issues/343
-  devtool: 'eval-source-map',
+  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.