Remove unnecessary guard against missing plugins in preflight check

src/index.js

@@ -2,7 +2,7 @@ import * as babel from '@babel/core';
 import { createFilter } from 'rollup-pluginutils';
 import createPreflightCheck from './preflightCheck.js';
 import helperPlugin from './helperPlugin.js';
-import { escapeRegExpCharacters, warnOnce } from './utils.js';
+import { addBabelPlugin, escapeRegExpCharacters, warnOnce } from './utils.js';
 import { RUNTIME, EXTERNAL, HELPERS } from './constants.js';
 
 const unpackOptions = ({
@@ -106,7 +106,7 @@ function createBabelPluginFactory(customCallback = returnObject) {
 					}
 
 					if (helpers !== RUNTIME) {
-						transformOptions = { ...transformOptions, plugins: transformOptions.plugins.concat(helperPlugin) };
+						transformOptions = addBabelPlugin(transformOptions, helperPlugin);
 					}
 
 					const result = babel.transformSync(code, transformOptions);

src/preflightCheck.js

@@ -1,5 +1,6 @@
 import { transformSync } from '@babel/core';
 import { INLINE, RUNTIME, EXTERNAL } from './constants.js';
+import { addBabelPlugin } from './utils.js';
 
 const MODULE_ERROR =
 	'Rollup requires that your Babel configuration keeps ES6 module syntax intact. ' +
@@ -39,10 +40,7 @@ export default function createPreflightCheck() {
 			let check = transformed.code;
 
 			if (~check.indexOf('class ')) {
-				check = transformSync(inputCode, {
-					...options,
-					plugins: (options.plugins || []).concat(fallbackClassTransform),
-				}).code;
+				check = transformSync(inputCode, addBabelPlugin(options, fallbackClassTransform)).code;
 			}
 
 			if (

src/utils.js

@@ -1,3 +1,5 @@
+export const addBabelPlugin = (options, plugin) => ({ ...options, plugins: options.plugins.concat(plugin) });
+
 let warned = {};
 export function warnOnce(ctx, msg) {
 	if (warned[msg]) return;