Prepare for Uint8List SDK breaking change

A recent change to the Dart SDK updated `HttpClientResponse` to implement `Stream<Uint8List>` rather than implementing `Stream<List<int>>`. This forwards-compatible chnage updates calls to `Stream.transform(StreamTransformer)` to instead call the functionally equivalent `StreamTransformer.bind(Stream)` API, which puts the stream in a covariant position and thus causes the SDK change to be non-breaking. https://github.com/dart-lang/sdk/issues/36900

Prepare for Uint8List SDK breaking change
2dba367a · Todd Volkert · 21ef6cdd · 2dba367a
--- a/example/lib/pay_page.dart
+++ b/example/lib/pay_page.dart
@@ -42,7 +42,7 @@ class _PayPageState extends State<PayPage> {
              };
              var request = await h.getUrl(Uri.parse(_url));
              var response = await request.close();
-              var data = await response.transform(Utf8Decoder()).join();
+              var data = await Utf8Decoder().bind(response).join();
              Map<String, dynamic> result = json.decode(data);
              print(result['appid']);
              print(result["timestamp"]);