Remove all usages of this.fire() (Polymer Legacy)

Change-Id: Ib5915ddd435c9347d9e5beeb55cdefa6af8c144a
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts b/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts
index cc7bb98..6f35481 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts
@@ -26,6 +26,7 @@
 import {IronA11yAnnouncer} from '@polymer/iron-a11y-announcer/iron-a11y-announcer';
 import {FixIronA11yAnnouncer} from '../../../types/types';
 import {appContext} from '../../../services/app-context';
+import {fireIronAnnounce} from '../../../utils/event-util';
 
 @customElement('gr-diff-mode-selector')
 export class GrDiffModeSelector extends LegacyElementMixin(PolymerElement) {
@@ -66,13 +67,7 @@
       announcement = 'Changed diff view to side by side';
     }
     if (announcement) {
-      this.fire(
-        'iron-announce',
-        {
-          text: announcement,
-        },
-        {bubbles: true}
-      );
+      fireIronAnnounce(this, announcement);
     }
   }