fix: low FPS on Windows (#1279)

ui/flutter/windows/runner/main.cpp

@@ -65,6 +65,11 @@ int APIENTRY wWinMain(_In_ HINSTANCE instance, _In_opt_ HINSTANCE prev,
 
   flutter::DartProject project(L"data");
 
+  // TODO: Remove this.
+  // This forces Flutter to use a separate thread for Dart.
+  // This mode will be removed in a future version of Flutter.
+  project.set_ui_thread_policy(flutter::UIThreadPolicy::RunOnSeparateThread);
+
   std::vector<std::string> command_line_arguments =
       GetCommandLineArguments();