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

runtime/vm/assembler_ia32.h

Index: runtime/vm/assembler_ia32.h
diff --git a/runtime/vm/assembler_ia32.h b/runtime/vm/assembler_ia32.h
index 4b7c87558a7b4b86485dfccc0ea95a68c22cdfb2..aa5259fb84d2fafc239362840f572f517d2ad90f 100644
--- a/runtime/vm/assembler_ia32.h
+++ b/runtime/vm/assembler_ia32.h
@@ -302,6 +302,9 @@ class Assembler : public ValueObject {
   void popl(Register reg);
   void popl(const Address& address);
 
+  void pushal();
+  void popal();
+
   void movl(Register dst, const Immediate& src);
   void movl(Register dst, Register src);
 
@@ -520,6 +523,8 @@ class Assembler : public ValueObject {
   void StoreIntoObject(Register object,  // Object we are storing into.
                        const FieldAddress& dest,  // Where we are storing into.
                        Register value);  // Value we are storing.
+  void StoreNullIntoObject(Register object,
+                           const FieldAddress& dest);
 
   void DoubleNegate(XmmRegister d);
   void FloatNegate(XmmRegister f);