dynamite: Use universal_io to make it work on web

packages/dynamite/lib/src/openapi_builder.dart

@@ -61,11 +61,11 @@ class OpenAPIBuilder implements Builder {
         '// ignore_for_file: camel_case_types',
         '// ignore_for_file: public_member_api_docs',
         "import 'dart:convert';",
-        "import 'dart:io';",
         "import 'dart:typed_data';",
         '',
         "import 'package:cookie_jar/cookie_jar.dart';",
         "import 'package:json_annotation/json_annotation.dart';",
+        "import 'package:universal_io/io.dart';",
         '',
         "export 'package:cookie_jar/cookie_jar.dart';",
         '',