title: Flutter, Dart and FirebaseAuth: How do I link a sign to a user?
body: <p>I'm experimenting with Flutter, Dart, and FirebaseAuth. I've been able to successfully login with a Google sign in, but when I try to login with a Facebook sign-in I get told "<strong>An account already exists with the same email address but different sign-in credentials. Sign in using a provider associated with this email address.</strong>" </p>
  
  <p>This is because firebase auth is basing everything on the user's email and I've already signed in using Google. So it's not allowing a sign in with Facebook. </p>
  
  <p>In some code I did years ago I just got the user and use <code>.lineWithCredential(facebookAuthCred)</code> to complete the link up with the established Google account in Firebase. But with the Dart implementation this doesn't work because <code>FirebaseAuth.instance.currentUser()</code> is returning a <code>null</code>. The docs on the error coming back from <code>.signInWithCredential(facebookAuthCred)</code> tell me to do <code>.fetchSignInMethodForEmail(...)</code> except that without a login I don't have an email address to pass to it.</p>
  
  <p>So I'm in this situation. If a user re-starts the app and selects Facebook instead of Google, how can I link the accounts when I don't have a <code>FirebaseUser</code> nor an email address?</p>
  
  <p>The only thing I've seen on the net so far is people saying you have to get the user to log in with their Google account first, but Logging in Google just to then login Facebook is a really terrible UI experience. </p>
  
  <p>I'm struggling to figure this out.</p>