Revert "refactor(core): Removed readonly getters and changed to readonly (#19225)"

This reverts commit 2b84b86fc0.

packages/platform-browser/animations/src/animation_renderer.ts

@@ -114,14 +114,13 @@ export class AnimationRendererFactory implements RendererFactory2 {
 }
 
 export class BaseAnimationRenderer implements Renderer2 {
-  readonly data: {[key: string]: any};
-
   constructor(
       protected namespaceId: string, public delegate: Renderer2, public engine: AnimationEngine) {
     this.destroyNode = this.delegate.destroyNode ? (n) => delegate.destroyNode !(n) : null;
-    this.data = delegate.data;
   }
 
+  get data() { return this.delegate.data; }
+
   destroyNode: ((n: any) => void)|null;
 
   destroy(): void {