[gardening] Mark large_class_declaration_test Slow.

This is follow-up to https://dart.googlesource.com/sdk/+/008aa729c53eec41a949d61e3d9f225dc5cb9254.

Change-Id: I70a05011b6b6287c0be9b879fb581511e9141287
Reviewed-on: https://dart-review.googlesource.com/62621
Reviewed-by: RĂ©gis Crelier <regis@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>
diff --git a/tests/language_2/language_2.status b/tests/language_2/language_2.status
index fbca329..fdece1b 100644
--- a/tests/language_2/language_2.status
+++ b/tests/language_2/language_2.status
@@ -13,6 +13,9 @@
 [ $compiler != dart2analyzer ]
 switch_case_warn_test: Skip # Analyzer only, see language_analyzer2.status
 
+[ $mode == debug ]
+large_class_declaration_test: Slow, Pass
+
 [ $mode == product ]
 assertion_test: SkipByDesign # Requires checked mode.
 generic_test: SkipByDesign # Requires checked mode.