Prepare for upcoming change to HttpRequest and HttpClientResponse (dart-lang/markdown#254)

An upcoming change to the Dart SDK will change `HttpRequest` and
`HttpClientResponse` from implementing `Stream<List<int>>` to
implementing `Stream<Uint8List>`.

This forwards-compatible change prepares for that SDK breaking
change by casting the Stream to `List<int>` before transforming
it.

https://github.com/dart-lang/sdk/issues/36900