fix: web fullscreen (#9577)

Signed-off-by: fufesou <linlong1266@gmail.com>

flutter/lib/models/web_model.dart

@@ -166,4 +166,10 @@ class PlatformFFI {
 
   // just for compilation
   void syncAndroidServiceAppDirConfigPath() {}
+
+  void setFullscreenCallback(void Function(bool) fun) {
+    context["onFullscreenChanged"] = (bool v) {
+      fun(v);
+    };
+  }
 }