Switch to eval-source-map (facebook#4930)

Author: jasonLaster

config/webpack.config.dev.js

@@ -76,7 +76,7 @@ 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: 'cheap-module-source-map',
+  devtool: 'eval-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.

config/webpackDevServer.config.js

@@ -9,6 +9,7 @@
 'use strict';
 
 const errorOverlayMiddleware = require('react-dev-utils/errorOverlayMiddleware');
+const evalSourceMapMiddleware = require('react-dev-utils/evalSourceMapMiddleware');
 const noopServiceWorkerMiddleware = require('react-dev-utils/noopServiceWorkerMiddleware');
 const ignoredFiles = require('react-dev-utils/ignoredFiles');
 const config = require('./webpack.config.dev');
@@ -89,7 +90,10 @@ module.exports = function(proxy, allowedHost) {
     },
     public: allowedHost,
     proxy,
-    before(app) {
+    before(app, server) {
+      // This lets us fetch source contents from webpack for the error overlay
+      app.use(evalSourceMapMiddleware(server));
+
       // This lets us open files from the runtime error overlay.
       app.use(errorOverlayMiddleware());
       // This service worker file is effectively a 'no-op' that will reset any