MissingPluginException(No implementation found for method signInWithCredential on channel plugins.flutter.io/firebase_auth)

I had a really hard time in making Firebase work. I think it should be REALLY easy, as both Flutter and Firebase are Google products. Anyway, I had to use this code for the AppDelegate.m file in iOS:

#include "AppDelegate.h"
#include "GeneratedPluginRegistrant.h"

@import UIKit;
@import Firebase;

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application
didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
  [FIRApp configure];
  [GeneratedPluginRegistrant registerWithRegistry:self];
  return [super application:application didFinishLaunchingWithOptions:launchOptions]; // YES;
}
@end

Have you tried something like this already?