Merge branch '24927-custom-event-polyfill-test' into 'master'

Adds tests for Custom Event polyfill

## What does this MR do?
Adds tests for CustomEvent polyfill.

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
Closes #24927

See merge request !7996

changelogs/unreleased/24927-custom-event-polyfill-test.yml

+---
+title: Adds tests for custom event polyfill
+merge_request: 7996
+author:

spec/javascripts/lib/utils/custom_event_polyfill_spec.js.es6

+//= require lib/utils/custom_event_polyfill
+
+describe('Custom Event Polyfill', () => {
+  it('should be defined', () => {
+    expect(window.CustomEvent).toBeDefined();
+  });
+
+  it('should create a `CustomEvent` instance', () => {
+    const e = new window.CustomEvent('foo');
+
+    expect(e.type).toEqual('foo');
+    expect(e.bubbles).toBe(false);
+    expect(e.cancelable).toBe(false);
+    expect(e.detail).toBeFalsy();
+  });
+
+  it('should create a `CustomEvent` instance with a `details` object', () => {
+    const e = new window.CustomEvent('bar', { detail: { foo: 'bar' } });
+
+    expect(e.type).toEqual('bar');
+    expect(e.bubbles).toBe(false);
+    expect(e.cancelable).toBe(false);
+    expect(e.detail.foo).toEqual('bar');
+  });
+
+  it('should create a `CustomEvent` instance with a `bubbles` boolean', () => {
+    const e = new window.CustomEvent('bar', { bubbles: true });
+
+    expect(e.type).toEqual('bar');
+    expect(e.bubbles).toBe(true);
+    expect(e.cancelable).toBe(false);
+    expect(e.detail).toBeFalsy();
+  });
+
+  it('should create a `CustomEvent` instance with a `cancelable` boolean', () => {
+    const e = new window.CustomEvent('bar', { cancelable: true });
+
+    expect(e.type).toEqual('bar');
+    expect(e.bubbles).toBe(false);
+    expect(e.cancelable).toBe(true);
+    expect(e.detail).toBeFalsy();
+  });
+});