Cleanups. More mutator thread asserts.

runtime/vm/flow_graph_optimizer.cc

Index: runtime/vm/flow_graph_optimizer.cc
diff --git a/runtime/vm/flow_graph_optimizer.cc b/runtime/vm/flow_graph_optimizer.cc
index 564b58ff90558111dadd84441bec00ce71d1d65d..9240358c6b9ebb9bc7224bc442041e349605fe5d 100644
--- a/runtime/vm/flow_graph_optimizer.cc
+++ b/runtime/vm/flow_graph_optimizer.cc
@@ -4621,6 +4621,8 @@ void FlowGraphOptimizer::VisitStoreInstanceField(
       field.set_is_unboxing_candidate(false);
       if (Compiler::IsBackgroundCompilation()) {
         // Delay deoptimization of dependent code to the code installation time.
+        // The invalidation of the background compilation result occurs only
+        // when the deoptimization is triggered at code installation.
         flow_graph()->deoptimize_dependent_code().Add(&field);
       } else {
         field.DeoptimizeDependentCode();