Support onLoad event for FileReader

[maple135790]

In MDN, FileReader has a event called onLoad, but unfortunately in current main b55f343, there is no such event to listen to.

Current workaround would need to write a custom Extension Method, mimic the the onLoadEnd event which is currently inside main

extension on FileReader {
  Stream<ProgressEvent> get onLoad =>
      EventStreamProviders.loadEvent.forTarget(this);
}

[srujzs]

Thanks for filing! Feel free to add a PR adding what you wrote above to

web/web/lib/src/helpers/events/events.dart

Line 246 in af5de5e

extension FileReaderEventGEtters on FileReader {

.