title: firebase auth undefined class 'OAuthProvider'
body: <p>I'm trying to implement firebase auth with apple this example
  <a href="https://medium.com/@karlwhiteprivate/flutter-firebase-sign-in-with-apple-c99967df142f" rel="nofollow noreferrer">https://medium.com/@karlwhiteprivate/flutter-firebase-sign-in-with-apple-c99967df142f</a></p>
  
  <p>but I faced this issue</p>
  
  <blockquote>
    <p>undefined class 'OAuthProvider'.</p>
  </blockquote>
  
  <p>in this snippet </p>
  
  <pre><code>OAuthProvider oAuthProvider =
      new OAuthProvider(providerId: "apple.com");
  final AuthCredential credential = oAuthProvider.getCredential(
    idToken:
        String.fromCharCodes(appleIdCredential.identityToken),
    accessToken:
        String.fromCharCodes(appleIdCredential.authorizationCode),
  );
  </code></pre>