Gardening: Marked 3 tests in precomp debug as slow.

tests/co19/co19-runtime.status

Index: tests/co19/co19-runtime.status
diff --git a/tests/co19/co19-runtime.status b/tests/co19/co19-runtime.status
index 9c4faf60fdd6e47ed036209dcc2c6d97f2ffde7c..af1096328809cca5b128d38964c2c400eb4d6c60 100644
--- a/tests/co19/co19-runtime.status
+++ b/tests/co19/co19-runtime.status
@@ -189,6 +189,9 @@ Language/Libraries_and_Scripts/Parts/syntax_t06: Fail
 [ ($runtime == vm || $runtime == dart_precompiled ) && $mode == debug ]
 LibTest/core/List/List_class_A01_t02: Pass, Slow
 LibTest/isolate/Isolate/spawn_A04_t01: Pass, Fail  # Issue 28269
+Language/Types/Interface_Types/subtype_t28: Pass, Slow
+Language/Types/Interface_Types/subtype_t27: Pass, Slow
+LibTest/collection/ListMixin/ListMixin_class_A01_t02: Pass, Slow
 
 [ ($runtime == dart_precompiled || $runtime == dart_app) && $mode == debug ]
 Language/Libraries_and_Scripts/Imports/deferred_import_t02: Crash # Issue 27201