Accessing partial download data via StreamedRequest does not seem to work on web

[BananaNeil]

I implemented the code in this Stackoverflow answer: https://stackoverflow.com/a/61021018/937841

And on the web, I found that _response = await http.Client().send('GET", ...) seems to download the entire file before advancing.

The callback passed to _response.stream.listen( ... ) only gets called once with the entire file downloaded, and then onDone is called immediately - this makes it impossible to add a progress loader with incremental download percentages.

On macos, I see the incremental download percentages with no issue.

[natebosch]

Feature request for supporting streaming downloads on the web is at #595