The line where is going to crash:
GoogleSignInAccount googleUser = await _googleSignIn.signIn();
What have I tried:
keytool -list -v \
-alias androiddebugkey -keystore ~/.android/debug.keystore
debug.keystore from .android to MyProject/android
Adding into android/build.gradle those lines:
Adding into android/app/build.gradle this lines:
release-signing.properties under my project folder with those lines in it.
Also, I've searched in ALL StackOverflow question that I could found for this question, and none of them was useful for me.
My pubspec.yaml:
firebase_core: ^0.4.0+1
firebase_analytics: ^3.0.1
cloud_firestore: ^0.11.0+2
firebase_auth: ^0.11.1
google_sign_in: ^4.0.1+3
rxdart: ^0.22.0
Auth class:
class AuthService {
  final GoogleSignIn _googleSignIn = GoogleSignIn();
  final FirebaseAuth _auth = FirebaseAuth.instance;
  final Firestore _db = Firestore.instance;
  Observable<FirebaseUser> user;
  Observable<Map<String, dynamic>> profile;
  PublishSubject loading = PublishSubject();
  AuthService() {
    user = Observable(_auth.onAuthStateChanged);
    profile = user.switchMap((FirebaseUser user) {
      if (user != null) {
        return _db
            .collection('user')
            .document(user.uid)
            .snapshots()
            .map((snap) => snap.data);
      } else {
        return Observable.just({});
      }
    });
  }
  Future<FirebaseUser> googleSignIn() async {
    loading.add(true);
    GoogleSignInAccount googleUser = await _googleSignIn.signIn();
    GoogleSignInAuthentication googleAuth = await googleUser.authentication;
    // FirebaseUser user = await _auth.signInWithGoogle(
        // accessToken: googleAuth.accessToken, idToken: googleAuth.idToken);
    final AuthCredential credential = GoogleAuthProvider.getCredential(
      accessToken: googleAuth.accessToken,
      idToken: googleAuth.idToken,
    );
    FirebaseUser user = await _auth.signInWithCredential(credential);
    updateUserData(user);
    print("Sign in" + user.displayName);
    loading.add(false);
    return user;
  }
}
Main class:
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  AuthService authService = AuthService();
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blueGrey,
      ),
      home: Scaffold(
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              MaterialButton(
                child: Text("Log in with Google"),
                color: Colors.blueGrey,
                textColor: Colors.deepOrange,
                onPressed: () => authService.googleSignIn(),
              ),
              MaterialButton(
                child: Text("LogOut"),
                color: Colors.redAccent,
                textColor: Colors.purple,
                onPressed: () => authService.signOut(),
              ),
            ],
          ),
        ),
      ),
    );
  }
}
Now if any of you could point me in the right direction I would be very grateful.
Here are some stackoverflow links that I've tried already:
I didn't have luck with any of them, please let me know what do you think and how can I fix it.
The OP, @Mircea, specifically mentioned error 12500. This is how I overcame that error:
I have spent the last few days running into the same problem. I refactored my code dozens of times, added break points, reset the SHA-1, SHA-256 and did just about every other 'code' based solution I could find. I was constantly running into the error:
E/flutter (11935): [ERROR:flutter/lib/ui/ui_dart_state.cc(148)] Unhandled Exception: PlatformException(sign_in_failed, com.google.android.gms.common.api.ApiException: 12500: , null)
I was losing my mind so I decided to start with fresh eyes and move slowly through solutions one more time. I noticed an obscure post nestled deep in a thread that mentioned something about having a support email. It was not until I added a said support email that things "magically" started working. I've attached a screenshot below of the Firebase settings for my project.

In my case, I have not enabled Google Sign In method.

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