Initial support for length guards on final fields.

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 e818aa2b1e9e5a553523ae2b07d102e33b3c7432..4861571d119d9410dce831c5095ff705a2e9af43 100644
--- a/runtime/vm/flow_graph_builder.cc
+++ b/runtime/vm/flow_graph_builder.cc
@@ -2938,6 +2938,12 @@ 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::kNoLength);
+  }
+
   store_value = Bind(BuildStoreExprTemp(store_value));
   GuardFieldInstr* guard =
       new GuardFieldInstr(store_value,