Generate code for store buffer updates in open-coded object field stores.

runtime/vm/code_generator_ia32.cc

Index: runtime/vm/code_generator_ia32.cc
diff --git a/runtime/vm/code_generator_ia32.cc b/runtime/vm/code_generator_ia32.cc
index d279d4aba080daf8416dd33cd965f7205bcdf2e3..23379b47a98bb3ab4c08470d8fd599c8f8f941ac 100644
--- a/runtime/vm/code_generator_ia32.cc
+++ b/runtime/vm/code_generator_ia32.cc
@@ -863,11 +863,14 @@ void CodeGenerator::VisitSequenceNode(SequenceNode* node_sequence) {
           *parsed_function_.saved_context_var(), CTX, kNoRegister);
       const Immediate raw_null =
           Immediate(reinterpret_cast<intptr_t>(Object::null()));
-      __ movl(CTX, raw_null);
+      __ StoreIntoObjectNoBarrier(EAX,
+                                  FieldAddress(EAX, Context::parent_offset()),
+                                  raw_null);
+    } else {
+      // Chain the new context in EAX to its parent in CTX.
+      __ StoreIntoObject(EAX, FieldAddress(EAX, Context::parent_offset()), CTX);
     }
 
-    // Chain the new context in EAX to its parent in CTX.
-    __ StoreIntoObject(EAX, FieldAddress(EAX, Context::parent_offset()), CTX);
     // Set new context as current context.
     __ movl(CTX, EAX);
     set_context_level(scope->context_level());