Impl saveFile for web

lib/platform/path.dart

@@ -1,7 +1,9 @@
 import 'dart:async';
+import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 import 'package:logging/logging.dart';
 import '../utils.dart';
+import 'save_file.dart';
 
 final Logger _log = Logger("platformPath");
 
@@ -31,6 +33,9 @@ class Path {
   Future<void> saveFile(
       String filenameWithoutExtension, String mimeType, Uint8List bytes,
       {String dir = ""}) async {
+    if (kIsWeb) {
+      return saveFileWeb(bytes, mimeType, filenameWithoutExtension);
+    }
     return _safChannel.invokeMethod(
         "saveFile", [filenameWithoutExtension, dir, mimeType, bytes]);
   }

lib/platform/save_file.dart

@@ -0,0 +1 @@
+export 'save_file_none.dart' if (dart.library.html) 'save_file_web.dart';

lib/platform/save_file_none.dart

@@ -0,0 +1,6 @@
+import 'dart:typed_data';
+
+void saveFileWeb(
+    Uint8List data, String mimeType, String filenameWithoutExtension) {
+  throw UnimplementedError();
+}

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);
+}