Prefix for dart:ui multiview change (#128152)

For https://github.com/flutter/engine/pull/42493.

packages/flutter_test/test/platform_dispatcher_test.dart

@@ -286,7 +286,7 @@ class _FakeFlutterView extends Fake implements FlutterView {
   final Display? _display;
 
   @override
-  final Object viewId = 1;
+  final int viewId = 1;
 }
 
 class _FakePlatformDispatcher extends Fake implements PlatformDispatcher {