up and download data

lib/helpers/web_download_helper.dart

@@ -0,0 +1,15 @@
+import 'dart:convert';
+// ignore: avoid_web_libraries_in_flutter
+import 'dart:html' as html;
+
+void downloadJsonFileWeb(String jsonData, String fileName) {
+  final bytes = utf8.encode(jsonData);
+  final blob = html.Blob([bytes]);
+  final url = html.Url.createObjectUrlFromBlob(blob);
+
+  final anchor = html.AnchorElement(href: url)
+    ..setAttribute('download', fileName)
+    ..click();
+
+  html.Url.revokeObjectUrl(url);
+}

lib/helpers/web_download_helper_stub.dart

@@ -0,0 +1,3 @@
+void downloadJsonFileWeb(String jsonData, String fileName) {
+  throw UnsupportedError('Web download is only supported on web platform');
+}