title: Flutter: FirebaseAuth token error: The custom token format is incorrect. Please check the documentation
body: <p>I'm working with Line App Login in Flutter, I can get user token in Line API but I'm stuck what to do next.</p>
  
  <p>Right now I'm trying to use it in FirebaseAuth.instance.signInWithCustomToken(token:customToken), but it doesn't work. The error log says "PlatformException(ERROR_INVALID_CUSTOM_TOKEN, The custom token format is incorrect. Please check the documentation. [ Invalid assertion format. ], null)" are there a work around for this?</p>
  
  <pre><code>lineSignIn() async {
      _flutterLineLogin.startLogin(_onLoginSuccess, _onLoginError);
    }
  
    void _onLoginSuccess(dynamic value) {
    FirebaseAuth.instance.signInWithCustomToken(token:value['accessToken'].toString()).then((onValue) {
        if (onValue != null) {
   ///this should be success
        }
      }).catchError((onError) {
        print(onError.toString());
      });
    }
  
    void _onLoginError(Object error) {
      debugPrint("PlatformException: ${error}");
    }
  </code></pre>
  
  <p>All I need is to authenticate the token in firebase. thanks everyone in advance!</p>