title: Method for FacebookLogin is not defined
body: <p>I have been struggling with this for a while guys. 
  My flutter_facebook_package is successfully hooked up with the app, but for some reason <code>fbLogin.logInWithReadPermissions</code> shows:</p>
  
  <pre><code>[dart] The method 'logInWithReadPermissions' isn't defined for the class 'FacebookLogin'
  </code></pre>
  
  <p>Did I miss anything with getting the package or importing something more?</p>
  
  <pre><code>import 'package:flutter/material.dart';
  
  //facebook signUp
  import 'package:flutter_facebook_login/flutter_facebook_login.dart';
  import 'package:firebase_auth/firebase_auth.dart';
  
  class FacebookLogin extends StatefulWidget {
    @override
    _FacebookLoginState createState() =&gt; _FacebookLoginState();
  }
  
  class _FacebookLoginState extends State&lt;FacebookLogin&gt; {
  
    FacebookLogin fbLogin = new FacebookLogin();
  
    @override
    Widget build(BuildContext context) {
      return 
        Container(
          height: 40.0,
          child: Material(
            borderRadius: BorderRadius.circular(20.0),
            shadowColor: Colors.blueAccent,
            color: Color.fromARGB(255, 76, 112, 166),
            elevation: 7.0,
            child: GestureDetector(
              onTap: () {
                fbLogin.logInWithReadPermissions(['email', 'public_profile']).then((result) {
                  switch(result.status) {
                    case FacebookLoginStatus.loggedIn:
                    FirebaseAuth.instance.signInWithFacebook(
                      accessToken: result.accessToken.token
                    ).then((signedInUser) {
                      print('Sighned in as ${signedInUser.displayName}');
                      Navigator.of(context).pushReplacementNamed('/homepage');
                    }).catchError((e) {
                      print(e);
                    });
                  }
                })
                .catchError((e) {
                  print(e);
                });
              },
              child: Center(
                child: Text('Log in with facebook',
                  style: TextStyle(
                    color: Colors.white,
                    fontWeight: FontWeight.bold,
                    fontFamily: 'Monserrat'
                  )),
              ),
            ),
          ),
        );
    }
  }
  </code></pre>