1a.dart

  /**
   * BEFORE
   */
  static facebook(App app, HttpRequest request) {
    var code = Uri.encodeComponent(request.uri.queryParameters['code']);
    var appId = Uri.encodeComponent(config['authentication']['facebook']['appId']);
    var appSecret = Uri.encodeComponent(config['authentication']['facebook']['appSecret']);
    var callbackUrl = Uri.encodeComponent(config['authentication']['facebook']['url']);

    var url = 'https://graph.facebook.com/oauth/access_token?client_id=$appId&redirect_uri=$callbackUrl&client_secret=$appSecret&code=$code';
    return http.read(url).then((String contents) {
      // The contents look like this: access_token=USER_ACCESS_TOKEN&expires=NUMBER_OF_SECONDS_UNTIL_TOKEN_EXPIRES
      var parameters = QueryString.parse(contents);
      var accessToken = parameters['access_token'];

      // Try to gather the user info.
      return http.read('https://graph.facebook.com/me?access_token=$accessToken&fields=picture,first_name,last_name,gender,birthday,email,location');
    }).then((String userInfo) {
      Map facebookData = JSON.decode(userInfo);

      var facebookId = facebookData['id'];

      // Get the profile pictures.
      return app.profilePictureUtil.downloadFacebookProfilePicture(id: facebookId, user: facebookId).then((Response res) {
        Map pictures = res.data;

        facebookData['picture'] = pictures['original'];
        facebookData['pictureSmall'] = pictures['small'];

        return facebookData;
      });

    }).then((Map facebookData) {
      // Streamline some of this data so it's easier to work with later.
      facebookData['location'] = facebookData['location'] != null ? facebookData['location']['name'] : null;
      facebookData['firstName'] = facebookData['first_name']; facebookData.remove("first_name");
      facebookData['lastName'] = facebookData['last_name']; facebookData.remove("last_name");
      var facebookId = facebookData['facebookId'] = facebookData['id']; facebookData.remove("id");

      var user = new UserModel()
        ..username = facebookData['facebookId']
        ..facebookId = facebookData['facebookId']
        ..firstName = facebookData['firstName']
        ..lastName = facebookData['lastName']
        ..email = facebookData['email']
        ..location = facebookData['location']
        ..gender = facebookData['gender']
        ..picture = facebookData['picture']
        ..pictureSmall = facebookData['pictureSmall']
        ..disabled = true;

      // Check for a session cookie in the request.
      var sessionCookie = request.cookies.firstWhere((cookie) => cookie.name == 'session', orElse: () => null);

      // If there's an existing session cookie, use it. Else, create a new session id.
      String sessionId = (sessionCookie == null || sessionCookie.value == null) ? app.sessionManager.createSessionId() : sessionCookie.value;

      return findFacebookIndex(facebookId).then((Map facebookIndexData) {
        // Upon sign in with Facebook, we redirect as appropriate.
        request.response.statusCode = 302;
        request.response.headers.add(HttpHeaders.LOCATION, '/');

        if (facebookIndexData == null) {
          // Add the session to our index, and add a session cookie to the request.
          app.sessionManager.addSessionToIndex(sessionId, facebookId).then((sessionData) {
            // Store the Facebook ID in an index that references the associated username.
            Firebase.put('/facebook_index/$facebookId.json', {'username': '$facebookId'}, auth: sessionData['authToken']);

            // Store the user, and we can use the index to find it and set a different username later.
            Firebase.put('/users/$facebookId.json', user.toJson(), auth: sessionData['authToken']);

            // Do not add cookie to request if this is new user, as we leave disabled until approved.
            app.sessionManager.addSessionCookieToRequest(request, sessionId);
          });
        } else {
          // If we already know of this Facebook user, update with any new data.
          var username = facebookIndexData['username'];

          // Get the existing user's data so we can compare against it.
          // TODO: Handle edge case where index points to non-existent user.
          return Firebase.get('/users/$username.json').then((Map userData) {
            if (userData == null) return null;
            facebookData.forEach((k, v) {
              if (userData[k] == null) userData[k] = v;
            });
            return userData;
          }).then((userData) {
            // TODO: handle null.
            // Update the session index with a reference to this username.
            app.sessionManager.addSessionToIndex(sessionId, username).then((sessionData) {
              Firebase.patch('/users/$username.json', userData, auth: sessionData['authToken']);
              app.sessionManager.addSessionCookieToRequest(request, sessionId);
            });
          });
        }
      });
    });
  }

1b.dart

  /**
   * AFTER
   */
  static facebook(App app, HttpRequest request) async {
    var code = Uri.encodeComponent(request.uri.queryParameters['code']);
    var appId = Uri.encodeComponent(config['authentication']['facebook']['appId']);
    var appSecret = Uri.encodeComponent(config['authentication']['facebook']['appSecret']);
    var callbackUrl = Uri.encodeComponent(config['authentication']['facebook']['url']);

    var url = 'https://graph.facebook.com/oauth/access_token?client_id=$appId&redirect_uri=$callbackUrl&client_secret=$appSecret&code=$code';
    String contents = await http.read(url);

    // The contents look like this: access_token=USER_ACCESS_TOKEN&expires=NUMBER_OF_SECONDS_UNTIL_TOKEN_EXPIRES
    var parameters = QueryString.parse(contents);
    var accessToken = parameters['access_token'];

    // Try to gather the user info.
    String userInfo = await http.read('https://graph.facebook.com/me?access_token=$accessToken&fields=picture,first_name,last_name,gender,birthday,email,location');
    Map facebookData = JSON.decode(userInfo);
    var facebookId = facebookData['id'];

    // Get the profile pictures.
    Response findFacebookProfilePicture = await app.profilePictureUtil.downloadFacebookProfilePicture(id: facebookId, user: facebookId);
    Map pictures = findFacebookProfilePicture.data;

    facebookData['picture'] = pictures['original'];
    facebookData['pictureSmall'] = pictures['small'];

    // Streamline some of this data so it's easier to work with later.
    facebookData['location'] = facebookData['location'] != null ? facebookData['location']['name'] : null;
    facebookData['firstName'] = facebookData['first_name']; facebookData.remove("first_name");
    facebookData['lastName'] = facebookData['last_name']; facebookData.remove("last_name");

    facebookId = facebookData['facebookId'] = facebookData['id']; facebookData.remove("id");

    var user = new UserModel()
      ..username = facebookData['facebookId']
      ..facebookId = facebookData['facebookId']
      ..firstName = facebookData['firstName']
      ..lastName = facebookData['lastName']
      ..email = facebookData['email']
      ..location = facebookData['location']
      ..gender = facebookData['gender']
      ..picture = facebookData['picture']
      ..pictureSmall = facebookData['pictureSmall']
      ..disabled = true
      ..onboardingStatus = OnboardingStatus.temporaryUser;

    // Check for a session cookie in the request.
    var sessionCookie = request.cookies.firstWhere((cookie) => cookie.name == 'session', orElse: () => null);

    // If there's an existing session cookie, use it. Else, create a new session id.
    String sessionId = (sessionCookie == null || sessionCookie.value == null) ? app.sessionManager.createSessionId() : sessionCookie.value;

    Map facebookIndexData = await findFacebookIndex(facebookId);

    // Upon sign in with Facebook, we redirect as appropriate.
    request.response.statusCode = 302;
    request.response.headers.add(HttpHeaders.LOCATION, '/');

    if (facebookIndexData == null) {
      // Add the session to our index, and add a session cookie to the request.
      Map sessionData = await app.sessionManager.addSessionToIndex(sessionId, facebookId);

      // Store the Facebook ID in an index that references the associated username.
      Firebase.put('/facebook_index/$facebookId.json', {'username': '$facebookId'}, auth: sessionData['authToken']);

      // Store the user, and we can use the index to find it and set a different username later.
      Firebase.put('/users/$facebookId.json', user.toJson(), auth: sessionData['authToken']);

      // Do not add cookie to request if this is new user, as we leave disabled until approved.
      app.sessionManager.addSessionCookieToRequest(request, sessionId);

    } else {
      // If we already know of this Facebook user, update with any new data.
      var username = facebookIndexData['username'];

      // Get the existing user's data so we can compare against it.
      // TODO: Handle edge case where index points to non-existent user.
      Map userData = await Firebase.get('/users/$username.json');

      if (userData == null) return null;
      facebookData.forEach((k, v) {
        if (userData[k] == null) userData[k] = v;
      });

      // TODO: handle null.
      // Update the session index with a reference to this username.
      Map sessionData = await app.sessionManager.addSessionToIndex(sessionId, username);
      Firebase.patch('/users/$username.json', userData, auth: sessionData['authToken']);
      app.sessionManager.addSessionCookieToRequest(request, sessionId);
    }
  }