due to the change in #3373
const extensionGlob = `**/*{${paths.extensions.join(',')}}*`;
const packPaths = sync(join(paths.source, paths.entry, extensionGlob));
-const entryPacks = [].concat(packPaths).concat(localePackPaths);
+const entryPacks = [].concat(packPaths).concat(localePackPaths).filter(path => path !== join(paths.source, paths.entry, 'custom.js'));
const customApplicationStyle = resolve(join(paths.source, 'styles/custom.scss'));
const originalApplicationStyle = resolve(join(paths.source, 'styles/application.scss'));