Add chain in flush & destroy calls, to ensure the return type is

Author: EmilianoSanchez

splitio_web/lib/splitio_web.dart

@@ -678,7 +678,8 @@ class SplitioWeb extends SplitioPlatform {
       bucketingKey: bucketingKey,
     );
 
-    return client.flush().toDart;
+    // `then` chain to ensure the return type is `Future<void>`
+    return client.flush().toDart.then<void>((_) {});
   }
 
   @override
@@ -689,7 +690,7 @@ class SplitioWeb extends SplitioPlatform {
       bucketingKey: bucketingKey,
     );
 
-    return client.destroy().toDart;
+    return client.destroy().toDart.then<void>((_) {});
   }
 
   @override

splitio_web/test/splitio_web_test.dart

@@ -675,7 +675,6 @@ void main() {
           }));
     });
 
-    // @TODO validate full config with pluggable Browser SDK modules
     test('init with config: full config', () async {
       SplitioWeb _platform = SplitioWeb();