PlatformViewLink: update cached surface to be Widget type (#38639)

packages/flutter/lib/src/widgets/platform_view.dart

@@ -678,7 +678,7 @@ class _PlatformViewLinkState extends State<PlatformViewLink> {
   int _id;
   PlatformViewController _controller;
   bool _platformViewCreated = false;
-  PlatformViewSurface _surface;
+  Widget _surface;
 
   @override
   Widget build(BuildContext context) {

packages/flutter/test/widgets/platform_view_test.dart

@@ -1968,7 +1968,6 @@ void main() {
     testWidgets('PlatformViewLink Widget dispose', (WidgetTester tester) async {
       FakePlatformViewController disposedController;
       final PlatformViewLink platformViewLink = PlatformViewLink(createPlatformViewController: (PlatformViewCreationParams params){
-        params.onPlatformViewCreated(params.id);
         disposedController = FakePlatformViewController(params.id);
         params.onPlatformViewCreated(params.id);
         return disposedController;
@@ -2041,5 +2040,21 @@ void main() {
         ]),
       );
     });
+
+    testWidgets('PlatformViewLink can take any widget to return in the SurfaceFactory', (WidgetTester tester) async {
+      final PlatformViewLink platformViewLink = PlatformViewLink(createPlatformViewController: (PlatformViewCreationParams params){
+        params.onPlatformViewCreated(params.id);
+        return FakePlatformViewController(params.id);
+      }, surfaceFactory: (BuildContext context,PlatformViewController controller) {
+        return Container();
+      });
+
+      await tester.pumpWidget(platformViewLink);
+
+      final Container container = tester.allWidgets.firstWhere((Widget widget){
+        return widget is Container;
+      });
+      expect(container, isNotNull);
+    });
   });
 }