simplify hierarchy of ResponseFuture (#791)

* simplify hierarchy of ResponseFuture

* dartfmt

* update changelog

---------

Co-authored-by: Moritz <mosum@google.com>

CHANGELOG.md

@@ -7,6 +7,8 @@
 - Require package:http 1.4.0
 - Require package:lints 6.0.0
 - Require package:protobuf 4.1.0
+- Simplify hierarchy of `ResponseFuture` (no longer have a private class in the
+  type hierarchy)
 
 ## 4.1.0
 

lib/src/client/common.dart

@@ -42,9 +42,7 @@ abstract class Response {
 }
 
 /// A gRPC response producing a single value.
-class ResponseFuture<R> extends DelegatingFuture<R>
-    with _ResponseMixin<dynamic, R> {
-  @override
+class ResponseFuture<R> extends DelegatingFuture<R> implements Response {
   final ClientCall<dynamic, R> _call;
 
   static R _ensureOnlyOneResponse<R>(R? previous, R element) {
@@ -65,21 +63,25 @@ class ResponseFuture<R> extends DelegatingFuture<R>
             .fold<R?>(null, _ensureOnlyOneResponse)
             .then(_ensureOneResponse),
       );
-}
-
-/// A gRPC response producing a stream of values.
-class ResponseStream<R> extends StreamView<R> with _ResponseMixin<dynamic, R> {
-  @override
-  final ClientCall<dynamic, R> _call;
-
-  ResponseStream(this._call) : super(_call.response);
-
-  @override
-  ResponseFuture<R> get single => ResponseFuture(_call);
-}
-
-mixin _ResponseMixin<Q, R> implements Response {
-  ClientCall<Q, R> get _call;
+
+  @override
+  Future<Map<String, String>> get headers => _call.headers;
+
+  @override
+  Future<Map<String, String>> get trailers => _call.trailers;
+
+  @override
+  Future<void> cancel() => _call.cancel();
+}
+
+/// A gRPC response producing a stream of values.
+class ResponseStream<R> extends StreamView<R> implements Response {
+  final ClientCall<dynamic, R> _call;
+
+  ResponseStream(this._call) : super(_call.response);
+
+  @override
+  ResponseFuture<R> get single => ResponseFuture(_call);
 
   @override
   Future<Map<String, String>> get headers => _call.headers;