Removes CustomEvent polyfill and tests

app/assets/javascripts/lib/utils/custom_event_polyfill.js.es6

-/**
- * CustomEvent support for IE
- */
-if (typeof window.CustomEvent !== 'function') {
-  window.CustomEvent = function CustomEvent(e, params) {
-    const options = params || { bubbles: false, cancelable: false, detail: undefined };
-    const evt = document.createEvent('CustomEvent');
-    evt.initCustomEvent(e, options.bubbles, options.cancelable, options.detail);
-    return evt;
-  };
-  window.CustomEvent.prototype = window.Event.prototype;
-}

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();
-  });
-});