refactor ZenMode to ES module syntax

app/assets/javascripts/dispatcher.js

@@ -4,7 +4,6 @@
 /* global ShortcutsNavigation */
 /* global IssuableIndex */
 /* global ShortcutsIssuable */
-/* global ZenMode */
 /* global Milestone */
 /* global IssuableForm */
 /* global LabelsSelect */
@@ -54,6 +53,7 @@ import UsersSelect from './users_select';
 import RefSelectDropdown from './ref_select_dropdown';
 import GfmAutoComplete from './gfm_auto_complete';
 import ShortcutsBlob from './shortcuts_blob';
+import ZenMode from './zen_mode';
 import initSettingsPanels from './settings_panels';
 import initExperimentalFlags from './experimental_flags';
 import OAuthRememberMe from './oauth_remember_me';

app/assets/javascripts/issuable_form.js

 /* eslint-disable func-names, space-before-function-paren, no-var, prefer-rest-params, wrap-iife, no-use-before-define, no-useless-escape, no-new, quotes, object-shorthand, no-unused-vars, comma-dangle, no-alert, consistent-return, no-else-return, prefer-template, one-var, one-var-declaration-per-line, curly, max-len */
 /* global GitLab */
-/* global ZenMode */
 /* global Autosave */
 /* global dateFormat */
 /* global Pikaday */
 
 import UsersSelect from './users_select';
 import GfmAutoComplete from './gfm_auto_complete';
+import ZenMode from './zen_mode';
 
 (function() {
   this.IssuableForm = (function() {

app/assets/javascripts/main.js

@@ -162,7 +162,6 @@ import './users_select';
 import './version_check_image';
 import './visibility_select';
 import './wikis';
-import './zen_mode';
 
 // eslint-disable-next-line global-require, import/no-commonjs
 if (process.env.NODE_ENV !== 'production') require('./test_utils/');

app/assets/javascripts/zen_mode.js

@@ -35,7 +35,7 @@ window.Dropzone = Dropzone;
 // **Target** a.js-zen-leave
 //
 
-class ZenMode {
+export default class ZenMode {
   constructor() {
     this.active_backdrop = null;
     this.active_textarea = null;
@@ -93,5 +93,3 @@ class ZenMode {
     });
   }
 }
-
-window.ZenMode = ZenMode;

spec/javascripts/zen_mode_spec.js

 /* eslint-disable space-before-function-paren, no-var, one-var, one-var-declaration-per-line, object-shorthand, comma-dangle, no-return-assign, new-cap, max-len */
 /* global Dropzone */
 /* global Mousetrap */
-/* global ZenMode */
 
-import '~/zen_mode';
+import ZenMode from '~/zen_mode';
 
 (function() {
   var enterZen, escapeKeydown, exitZen;