Impl saveFile for web

lib/platform/save_file_web.dart

@@ -0,0 +1,28 @@
+// ignore: avoid_web_libraries_in_flutter
+import 'dart:html';
+import 'dart:typed_data';
+
+void saveFileWeb(
+    Uint8List data, String mimeType, String filenameWithoutExtension) {
+  final blob = Blob([data], mimeType);
+  final url = Url.createObjectUrlFromBlob(blob);
+  final a = document.createElement("a") as AnchorElement;
+  a.href = url;
+  var ext = "";
+  switch (mimeType) {
+    case "image/jpeg":
+      ext = ".jpg";
+      break;
+    case "image/png":
+      ext = ".png";
+      break;
+    case "image/gif":
+      ext = ".gif";
+      break;
+    default:
+      break;
+  }
+  a.download = "$filenameWithoutExtension$ext";
+  a.click();
+  Url.revokeObjectUrl(url);
+}