[Gardening] Mark io/file_lock_test as Slow on dartk-debug

tests/standalone/standalone.status

Index: tests/standalone/standalone.status
diff --git a/tests/standalone/standalone.status b/tests/standalone/standalone.status
index 5903912278d3b6829fb2694ab5a9385b6eed04b6..31ff83b39dfa94a3354e7618e6d3e95787ac88ee 100644
--- a/tests/standalone/standalone.status
+++ b/tests/standalone/standalone.status
@@ -241,6 +241,9 @@ package/scenarios/packages_file_strange_formatting/mixed_line_ends_test: Crash #
 package/scenarios/packages_option_only/packages_option_only_noimports_test: Crash # Issue 28645
 package/scenarios/packages_option_only/packages_option_only_test: Crash, CompileTimeError # Issue 28645
 
+[ $compiler == dartk && $mode == debug ]
+io/file_lock_test: Slow
+
 [ ($runtime == vm || $runtime == dart_precompiled) && $mode == debug && $builder_tag == asan ]
 io/file_lock_test: Skip  # Timeout.
 io/test_runner_test: Skip  # Timeout.