title: "NoSuchMethodError: tried to call a non-function, such as null: 'dart.global.firebase.auth'" when initializing auth() in flutter for web
body: <p>I'm trying to implement sign-in function with Firebase Authentication in Flutter for Web app.
  I can already use Firestore to store/ extract data in the same app without signing-in.</p>
  
  <p>When executing the app, it shows the error "NoSuchMethodError: tried to call a non-function, such as null: 'dart.global.firebase.auth'"</p>
  
  <p>I think the error is caused by calling <code>auth()</code> method.</p>
  
  <p>Can you please advise how I should fix it?
  Or, is Firebase Authentication not supported Flutter for web yet??</p>
  
  <p>Imported firebase.dart.
  <a href="https://firebase.google.com/docs/reference/js/firebase" rel="noreferrer">https://firebase.google.com/docs/reference/js/firebase</a></p>
  
  <p><code>import 'package:firebase/firebase.dart';</code></p>
  
  <p>Initialized an object with auth() and called sign-in method from the parameter.</p>
  
  <pre><code>final Auth _auth = auth();
  
  Future&lt;UserCredential&gt; signIn(String email, String password) async {
    final UserCredential user = await _auth.signInAndRetrieveDataWithEmailAndPassword(email, password);
    return user;
  }
  </code></pre>