свойство onProgress
Поток событий progress, обрабатываемых этим FileReader.
Реализация
Stream<ProgressEvent> get onProgress => progressEvent.forTarget(this);
© 2012 the Dart project authors
Licensed under the BSD 3-Clause "New" or "Revised" License.
https://api.dart.dev/stable/2.19.6/dart-html/FileReader/onProgress.html
