Calling Flush[Async]() on the writer will NOT flush the stream.
Calling Flush[Async]() in Razor will flush both the writer and the stream.
Our normal flow will be to flush the writer, but not the stream. This
avoids chunking, but allows us to do a WriteAsync on the stream as part of
the call to FlushAsync. This is done to avoid a synchronous write due to
Dispose calling Flush on the writer, which needs to call Write on the
stream.
See issue for extensive background.
