fix(cdk/drag-drop): resolve incompatibility with property minification (#28980)

Fixes that the `showPopover` call was being minified by Closure and preventing the popover from opening.

(cherry picked from commit 58fa9b7)

Author: crisbeto

src/cdk/drag-drop/preview-ref.ts

@@ -64,8 +64,9 @@ export class PreviewRef {
     parent.appendChild(this._wrapper);
 
     // The null check is necessary for browsers that don't support the popover API.
-    if (this._wrapper.showPopover) {
-      this._wrapper.showPopover();
+    // Note that we use a string access for compatibility with Closure.
+    if ('showPopover' in this._wrapper) {
+      this._wrapper['showPopover']();
     }
   }