Files.app: Fix broken js-comp by casting details panel to HTMLElement.

Review URL: https://codereview.chromium.org/944453002

Cr-Commit-Position: refs/heads/master@{#317082}

ui/file_manager/file_manager/foreground/js/import_controller.js

@@ -558,7 +558,9 @@ importer.RuntimeCommandWidget.prototype.setDetailsVisible_ = function(visible) {
   } else {
     this.detailsPanel_.className = 'hidden';
     // transition duration is 200ms. Let's wait for 400ms.
-    ensureTransitionEndEvent(this.detailsPanel_, 400);
+    ensureTransitionEndEvent(
+        /** @type {!HTMLElement} */ (this.detailsPanel_),
+        400);
   }
 };