fix(cdk/drag-drop): remove preview after animate to placeholder animation completes (#29439)

* fix(cdk/drag-drop): remove preview after animate to placeholder animation completes

The comparison between the event target element and this._preview never returned true because this._preview is a class, and we need to compare its element instead. This regression was introduced in commit ebab924#diff-65b82ba5c290701d792881e9e389e8f1961848abb740bb20940d4383dfae4eacL121-R117.

* fixup! fix(cdk/drag-drop): remove preview after animate to placeholder animation completes

Fix linting error

(cherry picked from commit 5655661)

Author: skrtheboss

Diff for: src/cdk/drag-drop/drag-ref.ts

@@ -1100,7 +1100,9 @@ export class DragRef<T = any> {
         const handler = ((event: TransitionEvent) => {
           if (
             !event ||
-            (_getEventTarget(event) === this._preview && event.propertyName === 'transform')
+            (this._preview &&
+              _getEventTarget(event) === this._preview.element &&
+              event.propertyName === 'transform')
           ) {
             this._preview?.removeEventListener('transitionend', handler);
             resolve();

Diff for: src/cdk/drag-drop/preview-ref.ts

@@ -39,6 +39,10 @@ export class PreviewRef {
   /** Reference to the preview element. */
   private _preview: HTMLElement;
 
+  get element(): HTMLElement {
+    return this._preview;
+  }
+
   constructor(
     private _document: Document,
     private _rootElement: HTMLElement,