convert Compare class into ES module import/export syntax

app/assets/javascripts/compare.js

 /* eslint-disable func-names, space-before-function-paren, wrap-iife, quotes, no-var, object-shorthand, consistent-return, no-unused-vars, comma-dangle, vars-on-top, prefer-template, max-len */
 
-window.Compare = class Compare {
+export default class Compare {
   constructor(opts) {
     this.opts = opts;
     this.source_loading = $(".js-source-loading");
@@ -85,4 +85,4 @@ window.Compare = class Compare {
       }
     });
   }
-};
+}

app/assets/javascripts/dispatcher.js

@@ -22,7 +22,7 @@ import NewCommitForm from './new_commit_form';
 import Project from './project';
 import projectAvatar from './project_avatar';
 /* global MergeRequest */
-/* global Compare */
+import Compare from './compare';
 /* global CompareAutocomplete */
 /* global ProjectFindFile */
 import ProjectNew from './project_new';

app/assets/javascripts/main.js

@@ -40,7 +40,6 @@ import './admin';
 import './aside';
 import loadAwardsHandler from './awards_handler';
 import bp from './breakpoints';
-import './compare';
 import './compare_autocomplete';
 import './confirm_danger_modal';
 import Flash, { removeFlashClickListener } from './flash';