Convert gr-error-manager to typescript

The change converts the following files to typescript:

* elements/core/gr-error-manager/gr-error-manager.ts

Change-Id: Iafe5680b04b45ffe7b71fd9e1cc22f972e7f780b
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 bec07f0..e0333cc 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
@@ -27,6 +27,7 @@
 import {htmlTemplate} from './gr-diff-mode-selector_html';
 import {customElement, property} from '@polymer/decorators';
 import {IronA11yAnnouncer} from '@polymer/iron-a11y-announcer/iron-a11y-announcer';
+import {FixIronA11yAnnouncer} from '../../../types/types';
 
 export interface GrDiffModeSelector {
   $: {
@@ -34,10 +35,6 @@
   };
 }
 
-interface FixIronA11yAnnouncer extends IronA11yAnnouncer {
-  requestAvailability(): void;
-}
-
 @customElement('gr-diff-mode-selector')
 export class GrDiffModeSelector extends GestureEventListeners(
   LegacyElementMixin(PolymerElement)