fix(animations): renaming issue with DOMAnimation. (#21125)

Closure Compiler renames all properties that are "internal" to the
program. `DOMAnimation` however is external, it is a browser API, so its
fields must not be renamed.

This change marks `DOMAnimation` as external using `declare interface`,
which will cause Closure Compiler to back off and prevent renaming of
any of its fields.

PR Close #21125

packages/animations/browser/src/render/web_animations/dom_animation.ts

@@ -6,7 +6,15 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-export interface DOMAnimation {
+/**
+ * DOMAnimation represents the Animation Web API.
+ *
+ * It is an external API by the browser, and must thus use "declare interface",
+ * to prevent renaming by Closure Compiler.
+ *
+ * @see https://developer.mozilla.org/de/docs/Web/API/Animation
+ */
+export declare interface DOMAnimation {
   cancel(): void;
   play(): void;
   pause(): void;