Flash is now a ES6 module

Reduced the technical debt around our JS flash function by making it a
module that is imported rather than relying on the global function.
The global function still exists mainly for technical debt with how
some requests are being completed, but new JS should import the module
directly.

Also reduces some tech debt in the file by removing the need for jQuery.
Instead Flash is now 100% vanilla JS.

app/assets/javascripts/awards_handler.js

 /* eslint-disable class-methods-use-this */
-/* global Flash */
 import _ from 'underscore';
 import Cookies from 'js-cookie';
 import { isInIssuePage, updateTooltipTitle } from './lib/utils/common_utils';
+import Flash from './flash';
 const animationEndEventString = 'animationend webkitAnimationEnd MSAnimationEnd oAnimationEnd';
 const transitionEndEventString = 'transitionend webkitTransitionEnd oTransitionEnd MSTransitionEnd';

app/assets/javascripts/blob/balsamiq_viewer.js

-/* global Flash */
-
+import Flash from '../flash';
 import BalsamiqViewer from './balsamiq/balsamiq_viewer';
 function onError() {
-  const flash = new window.Flash('Balsamiq file could not be loaded.');
+  const flash = new Flash('Balsamiq file could not be loaded.');
   return flash;
 }

app/assets/javascripts/blob/file_template_mediator.js

 /* eslint-disable class-methods-use-this */
-/* global Flash */
-
+import Flash from '../flash';
 import FileTemplateTypeSelector from './template_selectors/type_selector';
 import BlobCiYamlSelector from './template_selectors/ci_yaml_selector';
 import DockerfileSelector from './template_selectors/dockerfile_selector';

app/assets/javascripts/blob/viewer/index.js

-/* global Flash */
+import Flash from '../../flash';
 import { handleLocationHash } from '../../lib/utils/common_utils';
 export default class BlobViewer {

app/assets/javascripts/boards/boards_bundle.js

 /* eslint-disable one-var, quote-props, comma-dangle, space-before-function-paren */
 /* global BoardService */
-/* global Flash */
 import _ from 'underscore';
 import Vue from 'vue';
 import VueResource from 'vue-resource';
+import Flash from '../flash';
 import FilteredSearchBoards from './filtered_search_boards';
 import eventHub from './eventhub';
 import './models/issue';

app/assets/javascripts/boards/components/board_sidebar.js

@@ -3,9 +3,9 @@
 /* global MilestoneSelect */
 /* global LabelsSelect */
 /* global Sidebar */
-/* global Flash */
 import Vue from 'vue';
+import Flash from '../../flash';
 import eventHub from '../../sidebar/event_hub';
 import AssigneeTitle from '../../sidebar/components/assignees/assignee_title';
 import Assignees from '../../sidebar/components/assignees/assignees';

app/assets/javascripts/boards/components/modal/footer.js

 /* eslint-disable no-new */
-/* global Flash */
 import Vue from 'vue';
+import Flash from '../../../flash';
 import './lists_dropdown';
 const ModalStore = gl.issueBoards.ModalStore;

app/assets/javascripts/boards/components/sidebar/remove_issue.js

 /* eslint-disable no-new */
-/* global Flash */
 import Vue from 'vue';
+import Flash from '../../../flash';
 const Store = gl.issueBoards.BoardsStore;

app/assets/javascripts/create_merge_request_dropdown.js

 /* eslint-disable no-new */
-/* global Flash */
+import Flash from './flash';
 import DropLab from './droplab/drop_lab';
 import ISetter from './droplab/plugins/input_setter';

app/assets/javascripts/cycle_analytics/cycle_analytics_bundle.js

@@ -2,6 +2,7 @@
 import Vue from 'vue';
 import Cookies from 'js-cookie';
+import Flash from '../flash';
 import Translate from '../vue_shared/translate';
 import banner from './components/banner.vue';
 import stageCodeComponent from './components/stage_code_component.vue';

app/assets/javascripts/deploy_keys/components/app.vue

 <script>
-  /* global Flash */
+  import Flash from '../../flash';
   import eventHub from '../eventhub';
   import DeployKeysService from '../service';
   import DeployKeysStore from '../store';

app/assets/javascripts/diff_notes/components/resolve_btn.js

 /* eslint-disable comma-dangle, object-shorthand, func-names, quote-props, no-else-return, camelcase, max-len */
 /* global CommentsStore */
 /* global ResolveService */
-/* global Flash */
 import Vue from 'vue';
+import Flash from '../../flash';
 const ResolveBtn = Vue.extend({
   props: {

app/assets/javascripts/diff_notes/services/resolve.js

-/* global Flash */
 /* global CommentsStore */
 import Vue from 'vue';
+import Flash from '../../flash';
 import '../../vue_shared/vue_resource_interceptor';
 window.gl = window.gl || {};

app/assets/javascripts/environments/components/environment.vue

 <script>
-/* global Flash */
 import Visibility from 'visibilityjs';
+import Flash from '../../flash';
 import EnvironmentsService from '../services/environments_service';
 import environmentTable from './environments_table.vue';
 import EnvironmentsStore from '../stores/environments_store';

app/assets/javascripts/environments/folder/environments_folder_view.vue

 <script>
-/* global Flash */
 import Visibility from 'visibilityjs';
+import Flash from '../../flash';
 import EnvironmentsService from '../services/environments_service';
 import environmentTable from '../components/environments_table.vue';
 import EnvironmentsStore from '../stores/environments_store';

app/assets/javascripts/filtered_search/dropdown_emoji.js

-/* global Flash */
-
-import Ajax from '~/droplab/plugins/ajax';
-import Filter from '~/droplab/plugins/filter';
+import Flash from '../flash';
+import Ajax from '../droplab/plugins/ajax';
+import Filter from '../droplab/plugins/filter';
 import './filtered_search_dropdown';
 class DropdownEmoji extends gl.FilteredSearchDropdown {

app/assets/javascripts/filtered_search/dropdown_non_user.js

-/* global Flash */
-
-import Ajax from '~/droplab/plugins/ajax';
-import Filter from '~/droplab/plugins/filter';
+import Flash from '../flash';
+import Ajax from '../droplab/plugins/ajax';
+import Filter from '../droplab/plugins/filter';
 import './filtered_search_dropdown';
 class DropdownNonUser extends gl.FilteredSearchDropdown {

app/assets/javascripts/filtered_search/dropdown_user.js

-/* global Flash */
-
-import AjaxFilter from '~/droplab/plugins/ajax_filter';
+import Flash from '../flash';
+import AjaxFilter from '../droplab/plugins/ajax_filter';
 import './filtered_search_dropdown';
 import { addClassIfElementExists } from '../lib/utils/dom_utils';

app/assets/javascripts/filtered_search/filtered_search_manager.js

+import Flash from '../flash';
 import FilteredSearchContainer from './container';
 import RecentSearchesRoot from './recent_searches_root';
 import RecentSearchesStore from './stores/recent_searches_store';
@@ -36,7 +37,7 @@ class FilteredSearchManager {
       .catch((error) => {
         if (error.name === 'RecentSearchesServiceError') return undefined;
         // eslint-disable-next-line no-new
-        new window.Flash('An error occurred while parsing recent searches');
+        new Flash('An error occurred while parsing recent searches');
         // Gracefully fail to empty array
         return [];
       })

app/assets/javascripts/filtered_search/filtered_search_visual_tokens.js

 import AjaxCache from '../lib/utils/ajax_cache';
-import '../flash'; /* global Flash */
+import Flash from '../flash';
 import FilteredSearchContainer from './container';
 import UsersCache from '../lib/utils/users_cache';