✅ Add test

dio/test/cancel_token_test.dart

@@ -1,3 +1,5 @@
+import 'dart:typed_data' show Uint8List;
+
 import 'package:dio/dio.dart';
 import 'package:dio/src/adapters/io_adapter.dart';
 import 'package:dio_test/util.dart';
@@ -112,4 +114,66 @@ void main() {
       expect(walkThroughHandlers, isFalse);
     });
   });
+
+  test('deallocates HttpClientRequest', () async {
+    final client = MockHttpClient();
+    final dio = Dio();
+    dio.httpClientAdapter = IOHttpClientAdapter(
+      createHttpClient: () => client,
+    );
+    final token = CancelToken();
+    final requests = <MockHttpClientRequest>{};
+    final requestsReferences = <WeakReference<MockHttpClientRequest>>{};
+    when(client.openUrl(any, any)).thenAnswer((_) async {
+      final request = MockHttpClientRequest();
+      requests.add(request);
+      requestsReferences.add(WeakReference(request));
+      when(request.close()).thenAnswer((_) async {
+        final response = MockHttpClientResponse();
+        when(response.headers).thenReturn(MockHttpHeaders());
+        when(response.statusCode).thenReturn(200);
+        when(response.reasonPhrase).thenReturn('OK');
+        when(response.isRedirect).thenReturn(false);
+        when(response.redirects).thenReturn([]);
+        when(response.cast())
+            .thenAnswer((_) => const Stream<Uint8List>.empty());
+        await Future.delayed(const Duration(milliseconds: 200));
+        return response;
+      });
+      when(request.abort()).thenAnswer((realInvocation) {
+        requests.remove(request);
+      });
+      return request;
+    });
+
+    final futures = [
+      dio.get('https://does.not.exists', cancelToken: token),
+      dio.get('https://does.not.exists', cancelToken: token),
+    ];
+    for (final future in futures) {
+      expectLater(
+        future,
+        throwsDioException(DioExceptionType.cancel),
+      );
+    }
+
+    // Opening requests.
+    await Future.delayed(const Duration(milliseconds: 100));
+    token.cancel();
+    // Aborting requests.
+    await Future.delayed(const Duration(seconds: 1));
+    expect(requests, isEmpty);
+
+    try {
+      await Future.wait(futures);
+    } catch (_) {
+      // Waiting here until all futures are completed.
+    }
+    expect(requests, isEmpty);
+    expect(requestsReferences, hasLength(2));
+
+    // GC.
+    List.generate(1 * 1024 * 1024, (index) => Object());
+    expect(requestsReferences.every((e) => e.target == null), isTrue);
+  });
 }