fix missing_return warnings (#702)

Author: a14n

packages/cloud_firestore/lib/src/firestore.dart

@@ -28,7 +28,7 @@ class Firestore {
 
   Firestore({FirebaseApp app}) : this.app = app ?? FirebaseApp.instance {
     if (_initialized) return;
-    channel.setMethodCallHandler((MethodCall call) {
+    channel.setMethodCallHandler((MethodCall call) async {
       if (call.method == 'QuerySnapshot') {
         final QuerySnapshot snapshot =
             new QuerySnapshot._(call.arguments, this);

packages/firebase_admob/test/firebase_admob_test.dart

@@ -30,6 +30,7 @@ void main() {
             return new Future<bool>.value(true);
           default:
             assert(false);
+            return null;
         }
       });
     });

packages/firebase_core/test/firebase_core_test.dart

@@ -56,7 +56,7 @@ void main() {
               },
             ];
           default:
-            break;
+            return null;
         }
       });
       log.clear();

packages/firebase_database/test/firebase_database_test.dart

@@ -274,7 +274,7 @@ void main() {
             equals(<String, dynamic>{'fakeKey': 'updated fakeValue'}));
         expect(
           database.reference().child('foo').runTransaction(
-                (MutableData mutableData) {},
+                (MutableData mutableData) async => null,
                 timeout: const Duration(milliseconds: 0),
               ),
           throwsA(isInstanceOf<AssertionError>()),

packages/firebase_messaging/example/lib/main.dart

@@ -138,15 +138,15 @@ class _PushMessagingExampleState extends State<PushMessagingExample> {
   void initState() {
     super.initState();
     _firebaseMessaging.configure(
-      onMessage: (Map<String, dynamic> message) {
+      onMessage: (Map<String, dynamic> message) async {
         print("onMessage: $message");
         _showItemDialog(message);
       },
-      onLaunch: (Map<String, dynamic> message) {
+      onLaunch: (Map<String, dynamic> message) async {
         print("onLaunch: $message");
         _navigateToItemDetail(message);
       },
-      onResume: (Map<String, dynamic> message) {
+      onResume: (Map<String, dynamic> message) async {
         print("onResume: $message");
         _navigateToItemDetail(message);
       },

packages/firebase_messaging/test/firebase_messaging_test.dart

@@ -102,11 +102,11 @@ void main() {
     final Completer<dynamic> onLaunch = new Completer<dynamic>();
     final Completer<dynamic> onResume = new Completer<dynamic>();
 
-    firebaseMessaging.configure(onMessage: (dynamic m) {
+    firebaseMessaging.configure(onMessage: (dynamic m) async {
       onMessage.complete(m);
-    }, onLaunch: (dynamic m) {
+    }, onLaunch: (dynamic m) async {
       onLaunch.complete(m);
-    }, onResume: (dynamic m) {
+    }, onResume: (dynamic m) async {
       onResume.complete(m);
     });
     final dynamic handler =

packages/firebase_storage/test/firebase_storage_test.dart

@@ -275,16 +275,14 @@ void main() {
                 return <String, String>{
                   'name': 'image.jpg',
                 };
-                break;
               case 'StorageReference#updateMetadata':
                 return <String, dynamic>{
                   'name': 'image.jpg',
                   'contentLanguage': 'en',
                   'customMetadata': <String, String>{'activity': 'test'},
                 };
-                break;
               default:
-                break;
+                return null;
             }
           });
           ref =

packages/google_maps_flutter/lib/src/controller.dart

@@ -75,7 +75,7 @@ class GoogleMapController extends ChangeNotifier {
   static Future<void> init() async {
     await _channel.invokeMethod('init');
     _controllers.clear();
-    _channel.setMethodCallHandler((MethodCall call) {
+    _channel.setMethodCallHandler((MethodCall call) async {
       final int mapId = call.arguments['map'];
       final GoogleMapController controller = _controllers[mapId];
       if (controller != null) {

packages/quick_actions/lib/quick_actions.dart

@@ -42,7 +42,7 @@ class QuickActions {
   ///
   /// Call this once before any further interaction with the the plugin.
   void initialize(QuickActionHandler handler) {
-    _kChannel.setMethodCallHandler((MethodCall call) {
+    _kChannel.setMethodCallHandler((MethodCall call) async {
       assert(call.method == 'launch');
       handler(call.arguments);
     });

packages/share/test/share_test.dart

@@ -16,7 +16,7 @@ void main() {
   setUp(() {
     mockChannel = new MockMethodChannel();
     // Re-pipe to mockito for easier verifies.
-    Share.channel.setMockMethodCallHandler((MethodCall call) {
+    Share.channel.setMockMethodCallHandler((MethodCall call) async {
       mockChannel.invokeMethod(call.method, call.arguments);
     });
   });