environment.plugins.get('Manifest').opts is now environment.plugins.get('Manifest').options. (#2434)

Author: mrsweaters

docs/testing.md

@@ -32,7 +32,7 @@ It is beneficial to use the same webpack configuration file (generated by webpac
 ```js
 // config/webpack/test.js
 const environment = require('./environment')
-environment.plugins.get('Manifest').opts.writeToFileEmit = process.env.NODE_ENV !== 'test'
+environment.plugins.get('Manifest').options.writeToFileEmit = process.env.NODE_ENV !== 'test'
 environment.loaders.append('istanbul-instrumenter', {
   test: /\.ts$/,
   enforce: "post",