Tests for GuardField length check along with bug fixes

runtime/vm/flow_graph_builder.cc

Index: runtime/vm/flow_graph_builder.cc
diff --git a/runtime/vm/flow_graph_builder.cc b/runtime/vm/flow_graph_builder.cc
index 2ff3dc1259ee4226fd760afe3bcfad46d4a6d637..e96a71c4d9addf4d08034148f43e388cb09375a4 100644
--- a/runtime/vm/flow_graph_builder.cc
+++ b/runtime/vm/flow_graph_builder.cc
@@ -3004,12 +3004,6 @@ void EffectGraphVisitor::VisitStoreInstanceFieldNode(
                                        dst_name);
   }
 
-  if (!node->field().is_final()) {
-    // For now, disable list length guarding on non-final fields by specifying
-    // that the field doesn't have a length. See issue #12485.
-    node->field().set_guarded_list_length(Field::kNoFixedLength);
-  }
-
   store_value = Bind(BuildStoreExprTemp(store_value));
   GuardFieldInstr* guard =
       new GuardFieldInstr(store_value,