refactor TreeView class to ES module syntax

app/assets/javascripts/dispatcher.js

@@ -9,7 +9,6 @@
 /* global MilestoneSelect */
 /* global Commit */
 /* global NotificationsForm */
-/* global TreeView */
 /* global NotificationsDropdown */
 /* global GroupAvatar */
 /* global LineHighlighter */
@@ -52,6 +51,7 @@ import UsersSelect from './users_select';
 import RefSelectDropdown from './ref_select_dropdown';
 import GfmAutoComplete from './gfm_auto_complete';
 import ShortcutsBlob from './shortcuts_blob';
+import TreeView from './tree';
 import UsagePing from './usage_ping';
 import UsernameValidator from './username_validator';
 import VersionCheckImage from './version_check_image';

app/assets/javascripts/main.js

@@ -153,7 +153,6 @@ import './subscription_select';
 import './syntax_highlight';
 import './task_list';
 import './todos';
-import './tree';
 import './user';
 
 // eslint-disable-next-line global-require, import/no-commonjs

app/assets/javascripts/tree.js

 /* eslint-disable func-names, space-before-function-paren, wrap-iife, max-len, quotes, consistent-return, no-var, one-var, one-var-declaration-per-line, no-else-return, prefer-arrow-callback, class-methods-use-this */
 
-class TreeView {
+export default class TreeView {
   constructor() {
     this.initKeyNav();
     // Code browser tree slider
@@ -62,5 +62,3 @@ class TreeView {
     });
   }
 }
-
-window.TreeView = TreeView;