Unable to send data via rest api about flutter_login_facebook HOT 2 CLOSED

I have figured out, token is inside token variable(FacebookAccessToken instance).
var tokenId = token.token;

from flutter_login_facebook.

I have also tried this-

    final Map<String, dynamic> params = {
      "access_token": token,
      "token_type": "bearer",
      "expires_in": 9183946
    };
    final http.Response response = await http.post(url, body: jsonEncode(params));

But still getting Error- JsonUnsupportedObjectError (Converting object to an encodable object failed: Instance of 'FacebookAccessToken')

from flutter_login_facebook.