Remove HttpClientRequest.response, as it's identical to HttpClientRequest.done.

tests/standalone/io/http_headers_state_test.dart

Index: tests/standalone/io/http_headers_state_test.dart
diff --git a/tests/standalone/io/http_headers_state_test.dart b/tests/standalone/io/http_headers_state_test.dart
index f2801baac463a7cb9d5f90b66a085a4fcb032328..db4cdc1ac9c631f74a920f1db6471d8d273518d5 100644
--- a/tests/standalone/io/http_headers_state_test.dart
+++ b/tests/standalone/io/http_headers_state_test.dart
@@ -53,7 +53,7 @@ void test(int totalConnections, [String body]) {
           // Cannot mutate request headers when data has been sent.
           Expect.throws(() => request.headers.add("X-Request-Header", "value3"),
                         (e) => e is HttpException);
-          return request.response;
+          return request.done;
         })
         .then((HttpClientResponse response) {
           // Cannot mutate response headers.