test: simple end-to-end verification

packages/@lwc/engine/src/framework/upgrade.ts

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: MIT
  * For full license text, see the LICENSE file in the repo root or https://opensource.org/licenses/MIT
  */
+import { ENABLE_FOO } from '@lwc/features';
 import assert from '../shared/assert';
 import { isUndefined, assign, isNull, isObject, isFunction, toString } from '../shared/language';
 import { createVM, removeRootVM, appendRootVM, getCustomElementVM, VMState } from './vm';
@@ -78,6 +79,9 @@ interface CreateElementOptions {
  * then it throws a TypeError.
  */
 export function createElement(sel: string, options: CreateElementOptions): HTMLElement {
+    if (ENABLE_FOO) {
+        throw new Error('this feature throws an error');
+    }
     if (!isObject(options) || isNull(options)) {
         throw new TypeError(
             `"createElement" function expects an object as second parameter but received "${toString(

packages/integration-karma/test/template/directive-if/index.spec.js

@@ -1,9 +1,12 @@
 import { createElement } from 'lwc';
+import { enableFeature } from 'features';
 import XTest from 'x/test';
 import XSlotted from 'x/slotted';
 import NestedRenderConditional from 'x/nestedRenderConditional';
 
 describe('if:true directive', () => {
+    enableFeature('ENABLE_FOO');
+
     it('should render if the value is truthy', () => {
         const elm = createElement('x-test', { is: XTest });
         elm.isVisible = true;