// create an httpClient compatible client
final innerClient = await IoCompatibleClient.create();
// use a method only available in httpClient
// there is no callback equivalent in RhttpCompatibleClient to get the cert of a response.
innerClient.badCertificateCallback = (X509Certificate cert, String url, int port) {
     // do something with the cert then return bool
     return false;
    };
// convert client to http.Client
final client = IOClient(innerclient);
// transfer it in a function that takes a http.Client and call client.post(url:..., headers:..., body:...)
MethodUsingPackageHttp(client);
client.post(url).