Wait for server response in http_body_test before closing client and server.

pkg/http_server/test/http_body_test.dart

Index: pkg/http_server/test/http_body_test.dart
diff --git a/pkg/http_server/test/http_body_test.dart b/pkg/http_server/test/http_body_test.dart
index 6c9efefafa9e64492cba1510bf162115c6f27dc6..b65d97a8d93c5b20b4f5555e14ec28a9f1309272 100644
--- a/pkg/http_server/test/http_body_test.dart
+++ b/pkg/http_server/test/http_body_test.dart
@@ -155,7 +155,9 @@ void testHttpServerRequestBody() {
             if (shouldFail) {
               expect(response.statusCode, equals(HttpStatus.BAD_REQUEST));
             }
-            response.fold(null, (x, y) {});
+            return response.drain();
+          )}
+          .then((_) {
             client.close();
             server.close();
           });