Use decoder.bind(stream) instead of stream.transform(decoder)

This updates the implementation of `Encoding.decodeStream()` to
use `decoder.bind(stream)` rather than `stream.transform(decoder)`
in order to put the stream in a covariant position.

This in turn allows callers to use `decodeStream` on cases that
recently changed from `Stream<List<int>>` to `Stream<Uint8List>`
(such as `HttpClientResponse` or `File.openRead()`) without breaking
those call sites.

Bug: #36900
Change-Id: Id98fa7f88f0870c46371903ec47300af2091e613
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/108161
Reviewed-by: William Hesse <[email protected]>
Commit-Queue: Todd Volkert <[email protected]>

Author: tvolkert

sdk/lib/convert/encoding.dart

@@ -19,8 +19,8 @@ abstract class Encoding extends Codec<String, List<int>> {
   Converter<List<int>, String> get decoder;
 
   Future<String> decodeStream(Stream<List<int>> byteStream) {
-    return byteStream
-        .transform<String>(decoder)
+    return decoder
+        .bind(byteStream)
         .fold(StringBuffer(),
             (StringBuffer buffer, String string) => buffer..write(string))
         .then((StringBuffer buffer) => buffer.toString());