Watch for changes in src/**/node_modules (facebook#3230)

* Allow the dev server to watch for changes in src/node_modules

* fix eslint error

* fix broken regex

* handle trailing slash edge case for file paths

Closes facebook#2760
Fixes facebook#3223

config/webpackDevServer.config.js

@@ -10,6 +10,7 @@
 
 const errorOverlayMiddleware = require('react-dev-utils/errorOverlayMiddleware');
 const noopServiceWorkerMiddleware = require('react-dev-utils/noopServiceWorkerMiddleware');
+const path = require('path');
 const config = require('./webpack.config.dev');
 const paths = require('./paths');
 
@@ -72,8 +73,13 @@ module.exports = function(proxy, allowedHost) {
     quiet: true,
     // Reportedly, this avoids CPU overload on some systems.
     // https://github.com/facebookincubator/create-react-app/issues/293
+    // src/node_modules is not ignored to support absolute imports
+    // https://github.com/facebookincubator/create-react-app/issues/1065
     watchOptions: {
-      ignored: /node_modules/,
+      ignored: new RegExp(
+        `^(?!${path.normalize(paths.appSrc + '/')}).+[\\/]node_modules[\\/]`,
+        'g'
+      ),
     },
     // Enable HTTPS if the HTTPS environment variable is set to 'true'
     https: protocol === 'https',