Prepare for upcoming change to File.openRead() (#125)

An upcoming change to the Dart SDK will change the signature
of `File.openRead()` from returning `Stream<List<int>>` to
returning `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