title: Obtain a new token by refresh token with google_sign_in Flutter
body: <p>I'm writing an application that call google fit rest api from Flutter. </p>
  
  <p>I need to sign with google using (<a href="https://pub.dev/packages/google_sign_in" rel="noreferrer">https://pub.dev/packages/google_sign_in</a>).
  I can obtain a token without problem (see <a href="https://stackoverflow.com/questions/52458968/did-anyone-manage-to-get-the-id-token-from-google-sign-in-flutter">Did anyone manage to get the id token from google sign in (Flutter)</a>) but how to obtain a new token when it is expired?</p>
  
  <p>I dont' want to ask to the user to login and obtain a new token every hour</p>