[firebase_messaging] Add support for stable Instance ID.

packages/firebase_messaging/android/src/main/java/io/flutter/plugins/firebasemessaging/FirebaseMessagingPlugin.java

@@ -248,6 +248,22 @@ public void onComplete(@NonNull Task<InstanceIdResult> task) {
                   result.success(task.getResult().getToken());
                 }
               });
+    } else if ("getInstanceID".equals(call.method)) {
+      FirebaseInstanceId.getInstance()
+          .getInstanceId()
+          .addOnCompleteListener(
+              new OnCompleteListener<InstanceIdResult>() {
+                @Override
+                public void onComplete(@NonNull Task<InstanceIdResult> task) {
+                  if (!task.isSuccessful()) {
+                    Log.w(TAG, "getInstanceID, error fetching instanceID: ", task.getException());
+                    result.success(null);
+                    return;
+                  }
+
+                  result.success(task.getResult().getId());
+                }
+              });
     } else if ("deleteInstanceID".equals(call.method)) {
       new Thread(
               new Runnable() {

packages/firebase_messaging/example/lib/main.dart

@@ -88,7 +88,8 @@ class PushMessagingExample extends StatefulWidget {
 }
 
 class _PushMessagingExampleState extends State<PushMessagingExample> {
-  String _homeScreenText = "Waiting for token...";
+  String _idStatusText = "Waiting for ID...";
+  String _tokenStatusText = "Waiting for token...";
   bool _topicButtonsDisabled = false;
 
   final FirebaseMessaging _firebaseMessaging = FirebaseMessaging();
@@ -159,12 +160,19 @@ class _PushMessagingExampleState extends State<PushMessagingExample> {
         .listen((IosNotificationSettings settings) {
       print("Settings registered: $settings");
     });
+    _firebaseMessaging.getId().then((id) {
+      assert(id != null);
+      setState(() {
+        _idStatusText = "Instance ID: $id";
+      });
+      print(_idStatusText);
+    });
     _firebaseMessaging.getToken().then((String token) {
       assert(token != null);
       setState(() {
-        _homeScreenText = "Push Messaging token: $token";
+        _tokenStatusText = "Push Messaging token: $token";
       });
-      print(_homeScreenText);
+      print(_tokenStatusText);
     });
   }
 
@@ -189,7 +197,10 @@ class _PushMessagingExampleState extends State<PushMessagingExample> {
           child: Column(
             children: <Widget>[
               Center(
-                child: Text(_homeScreenText),
+                child: Text(_idStatusText),
+              ),
+              Center(
+                child: Text(_tokenStatusText),
               ),
               Row(children: <Widget>[
                 Expanded(

packages/firebase_messaging/ios/Classes/FLTFirebaseMessagingPlugin.m

@@ -167,6 +167,17 @@ - (void)handleMethodCall:(FlutterMethodCall *)call result:(FlutterResult)result
             result(instanceIDResult.token);
           }
         }];
+  } else if ([@"getInstanceID" isEqualToString:method]) {
+    [[FIRInstanceID instanceID]
+        instanceIDWithHandler:^(FIRInstanceIDResult *_Nullable instanceIDResult,
+                                NSError *_Nullable error) {
+          if (error != nil) {
+            NSLog(@"getInstanceID, error fetching instanceID: %@", error);
+            result(nil);
+          } else {
+            result(instanceIDResult.instanceID);
+          }
+        }];
   } else if ([@"deleteInstanceID" isEqualToString:method]) {
     [[FIRInstanceID instanceID] deleteIDWithHandler:^void(NSError *_Nullable error) {
       if (error.code != 0) {

packages/firebase_messaging/lib/firebase_messaging.dart

@@ -150,6 +150,11 @@ class FirebaseMessaging {
     return await _channel.invokeMethod<String>('getToken');
   }
 
+  /// Returns the FCM stable ID.
+  Future<String> getId() async {
+    return await _channel.invokeMethod<String>('getInstanceID');
+  }
+
   /// Subscribe to topic in background.
   ///
   /// [topic] must match the following regular expression:

packages/firebase_messaging/test/firebase_messaging_test.dart

@@ -151,6 +151,11 @@ void main() {
     verify(mockChannel.invokeMethod<void>('unsubscribeFromTopic', myTopic));
   });
 
+  test('getId', () {
+    firebaseMessaging.getId();
+    verify(mockChannel.invokeMethod<String>('getInstanceID'));
+  });
+
   test('getToken', () {
     firebaseMessaging.getToken();
     verify(mockChannel.invokeMethod<String>('getToken'));