Fix flush return type and simplify promise handling

Author: EmilianoSanchez

splitio_web/lib/splitio_web.dart

@@ -695,10 +695,7 @@ class SplitioWeb extends SplitioPlatform {
       bucketingKey: bucketingKey,
     );
 
-    final result = client.flush();
-
-    // @TODO remove wrapping to Future<void> when JS SDK `flush` type is fixed
-    return result.toDart.then((_) {});
+    return client.flush().toDart;
   }
 
   @override
@@ -709,9 +706,7 @@ class SplitioWeb extends SplitioPlatform {
       bucketingKey: bucketingKey,
     );
 
-    final result = client.destroy();
-
-    return result.toDart;
+    return client.destroy().toDart;
   }
 
   @override

splitio_web/lib/src/js_interop.dart

@@ -83,7 +83,7 @@ extension type JS_IBrowserClient._(JSObject _) implements JSObject {
   external JSBoolean setAttributes(JSObject attributes);
   external JSObject getAttributes();
   external JSBoolean clearAttributes();
-  external JSPromise flush();
+  external JSPromise<Null> flush();
   external JSPromise<Null> destroy();
   external JSFunction on;
   external JSFunction off;