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 ce28a043b6ce700e8297ffc703f5f4a76b0d79c1..ca1ce26048dff3e652c69b5708d4fccd24c862f5 100644
--- a/runtime/vm/flow_graph_builder.cc
+++ b/runtime/vm/flow_graph_builder.cc
@@ -3010,12 +3010,6 @@ 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,