Add support for WritableRegister policy in the register allocator.

runtime/vm/assembler_ia32.cc

Index: runtime/vm/assembler_ia32.cc
diff --git a/runtime/vm/assembler_ia32.cc b/runtime/vm/assembler_ia32.cc
index dbba71ba15573a8d6a528d576e198817b8a2834a..807401c589f29d357eda86c58df69c4b60a17f2f 100644
--- a/runtime/vm/assembler_ia32.cc
+++ b/runtime/vm/assembler_ia32.cc
@@ -1422,10 +1422,9 @@ void Assembler::StoreIntoObjectFilter(Register object,
 void Assembler::StoreIntoObject(Register object,
                                 const FieldAddress& dest,
                                 Register value) {
-  // TODO(kmillikin): pass temp registers to avoid pushing registers.
+  ASSERT(object != value);
   movl(dest, value);
   Label done;
-  pushl(value);
   StoreIntoObjectFilter(object, value, &done);
   // A store buffer update is required.
   if (value != EAX) pushl(EAX);  // Preserve EAX.
@@ -1433,7 +1432,6 @@ void Assembler::StoreIntoObject(Register object,
   call(&StubCode::UpdateStoreBufferLabel());
   if (value != EAX) popl(EAX);  // Restore EAX.
   Bind(&done);
-  popl(value);
 }