How to get the ID token from FirebaseAuth

getIdToken() returns a Future - you have to await it to actually get the token string:

var token = await FirebaseAuth.instance.currentUser().getIdToken();
var response = await httpClient.get(url,headers: {'Authorization':"Bearer $token"});

U can also retrieve the id token from the window object like so:

const idToken = window.gapi.auth2.getAuthInstance().currentUser.get().getAuthResponse().id_token

I think that there is a bug in Flutter.

Now to call the method getIdToken() you need to do this:

FirebaseUser user = await FirebaseAuth.instance.currentUser();

String token;

user.getIdToken().then((result) {

    token = result.token;
});

You can't call FirebaseAuth.instance.currentUser().getIdToken()