Invalid token from google_sign_in plugin when verifying on server side - dart

Question

Please i am trying to implement social authentication on my flutter app with plugin google_sign_in plugin. It works properly on the client side and retrieves the account details of the user.
My problem is that i want to save the user in my database, so i need to verify the id_token gotten at the server side. I tried doing this with the google API library on the server side as shown here, but i key getting an invalid token result. I finally decided to try the tokeninfo endpoint https://www.googleapis.com/oauth2/v3/tokeninfo?id_token=<my_token> which should return 200 response with a body containing the user details and other info, but i get a 400 response with the body {"error_description": "Invalid Value"}.
I decoded the token with jwt.io to make sure it isn't an expired token and the token was still very much valid.
This is the google_sign_in code i used on flutter to get the id_token.
GoogleSignIn _googleSignIn = new GoogleSignIn(
scopes: <String>[
'profile',
'email',
'https://www.googleapis.com/auth/contacts.readonly',
],
);
Future<Null> _handleSignIn() async {
try {
_googleSignIn.signIn().then((result){
result.authentication.then((googleKey){
print(googleKey.accessToken);
print(googleKey.idToken);
print(_googleSignIn.currentUser.displayName);
}).catchError((err){
print('inner error');
});
}).catchError((err){
print('error occured');
});
print('signed in .....');
} catch (error) {
print(error);
}
}
Please any help would be greatly appreciated.

Answer 1

It turns out the tokens were valid all along. The tokens get truncated on the command line and become invalid.
The way I test the validity now is to use an HTTP package to test without having to copy the token.
eg.
Dio dio = new Dio();
Response response = await dio.get('https://www.googleapis.com/oauth2/v1/tokeninfo?id_token='+googleKey.idToken);
print(response.data); //contains the token info

Answer 2

In document google_sign_in they have detail for IOS, but in Android they have missing those step:
Add
classpath 'com.google.gms:google-services:4.2.0'
in dependencies in your root "build.gradle"
Add
apply plugin: 'com.google.gms.google-services'
in bottom your "app/build.gradle"
Add
implementation "com.google.gms:google-services:4.2.0"
in dependencies in "app/build.gradle"
Go to your firebase console, download
google-services.json
And copy it to directory "app/"
After complete all those step you can run and get idToken normal