adding support auth link for Twitter

* Link federated Twitter auth provider credentials to a user account.
* This fix calls method ```linkWithCredential()``` like other providers (facebook, google).

Author: ko2ic

packages/firebase_auth/android/src/main/java/io/flutter/plugins/firebaseauth/FirebaseAuthPlugin.java

@@ -111,6 +111,9 @@ public void onMethodCall(MethodCall call, Result result) {
       case "linkWithFacebookCredential":
         handleLinkWithFacebookCredential(call, result);
         break;
+      case "linkWithTwitterCredential":
+        handleLinkWithTwitterCredential(call, result);
+        break;
       case "updateProfile":
         handleUpdateProfile(call, result);
         break;
@@ -372,6 +375,18 @@ private void handleLinkWithFacebookCredential(MethodCall call, final Result resu
         .addOnCompleteListener(new SignInCompleteListener(result));
   }
 
+  private void handleLinkWithTwitterCredential(MethodCall call, final Result result) {
+    @SuppressWarnings("unchecked")
+    Map<String, String> arguments = (Map<String, String>) call.arguments;
+    String authToken = arguments.get("authToken");
+    String authTokenSecret = arguments.get("authTokenSecret");
+    AuthCredential credential = TwitterAuthProvider.getCredential(authToken, authTokenSecret);
+    firebaseAuth
+        .getCurrentUser()
+        .linkWithCredential(credential)
+        .addOnCompleteListener(new SignInCompleteListener(result));
+  }
+
   private void handleSignInWithFacebook(MethodCall call, final Result result) {
     @SuppressWarnings("unchecked")
     Map<String, String> arguments = (Map<String, String>) call.arguments;

packages/firebase_auth/ios/Classes/FirebaseAuthPlugin.m

@@ -176,6 +176,15 @@ - (void)handleMethodCall:(FlutterMethodCall *)call result:(FlutterResult)result
                                         completion:^(FIRUser *user, NSError *error) {
                                           [self sendResult:result forUser:user error:error];
                                         }];
+  } else if ([@"linkWithTwitterCredential" isEqualToString:call.method]) {
+    NSString *authToken = call.arguments[@"authToken"];
+    NSString *authTokenSecret = call.arguments[@"authTokenSecret"];
+    FIRAuthCredential *credential =
+        [FIRTwitterAuthProvider credentialWithToken:authToken secret:authTokenSecret];
+    [[FIRAuth auth].currentUser linkWithCredential:credential
+                                        completion:^(FIRUser *user, NSError *error) {
+                                          [self sendResult:result forUser:user error:error];
+                                        }];
   } else if ([@"updateProfile" isEqualToString:call.method]) {
     FIRUserProfileChangeRequest *changeRequest = [[FIRAuth auth].currentUser profileChangeRequest];
     if (call.arguments[@"displayName"]) {

packages/firebase_auth/lib/firebase_auth.dart

@@ -428,6 +428,21 @@ class FirebaseAuth {
     return currentUser;
   }
 
+  Future<FirebaseUser> linkWithTwitterCredential({
+    @required String authToken,
+    @required String authTokenSecret,
+  }) async {
+    final Map<dynamic, dynamic> data = await channel.invokeMethod(
+      'linkWithTwitterCredential',
+      <String, String>{
+        'authToken': authToken,
+        'authTokenSecret': authTokenSecret,
+      },
+    );
+    final FirebaseUser currentUser = new FirebaseUser._(data);
+    return currentUser;
+  }
+
   /// Sets the user-facing language code for auth operations that can be
   /// internationalized, such as [sendEmailVerification]. This language
   /// code should follow the conventions defined by the IETF in BCP47.

packages/firebase_auth/test/firebase_auth_test.dart

@@ -16,6 +16,8 @@ const String kMockEmail = 'test@example.com';
 const String kMockPassword = 'passw0rd';
 const String kMockIdToken = '12345';
 const String kMockAccessToken = '67890';
+const String kMockAuthToken = '23456';
+const String kMockAuthTokenSecret = '78901';
 const String kMockCustomToken = '12345';
 const String kMockPhoneNumber = '5555555555';
 const String kMockVerificationId = '12345';
@@ -247,6 +249,26 @@ void main() {
       );
     });
 
+    test('linkWithTwitterCredential', () async {
+      final FirebaseUser user = await auth.linkWithTwitterCredential(
+        authToken: kMockAuthToken,
+        authTokenSecret: kMockAuthTokenSecret,
+      );
+      verifyUser(user);
+      expect(
+        log,
+        <Matcher>[
+          isMethodCall(
+            'linkWithTwitterCredential',
+            arguments: <String, String>{
+              'authToken': kMockAuthToken,
+              'authTokenSecret': kMockAuthTokenSecret,
+            },
+          ),
+        ],
+      );
+    });
+
     test('signInWithFacebook', () async {
       final FirebaseUser user = await auth.signInWithFacebook(
         accessToken: kMockAccessToken,
@@ -378,7 +400,7 @@ void main() {
 
     test('updateEmail', () async {
       final String updatedEmail = 'atestemail@gmail.com';
-      auth.updateEmail(email: updatedEmail);
+      await auth.updateEmail(email: updatedEmail);
       expect(
         log,
         <Matcher>[