Append to descriptors over map.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index e48787b03856d7e24a6cd8e496f525d0bab94a29..d4f27dedf7ba1eaeef693abd703e0f6909843724 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2110,9 +2110,10 @@ void DescriptorArray::Set(int descriptor_number,
 }
 
 
-void DescriptorArray::Append(Descriptor* desc,
-                             const WhitenessWitness& witness) {
-  int descriptor_number = NumberOfSetDescriptors();
+int DescriptorArray::Append(Descriptor* desc,
+                            const WhitenessWitness& witness,
+                            int number_of_set_descriptors) {
+  int descriptor_number = number_of_set_descriptors;
   int enumeration_index = descriptor_number + 1;
   desc->SetEnumerationIndex(enumeration_index);
 
@@ -2128,7 +2129,7 @@ void DescriptorArray::Append(Descriptor* desc,
   }
 
   Set(descriptor_number, desc, witness);
-  SetLastAdded(descriptor_number);
+  return descriptor_number;
 }
 
 
@@ -3554,6 +3555,14 @@ void Map::ClearDescriptorArray(Heap* heap, WriteBarrierMode mode) {
 }
 
 
+void Map::AppendDescriptor(Descriptor* desc,
+                           const DescriptorArray::WhitenessWitness& witness) {
+  DescriptorArray* descriptors = instance_descriptors();
+  int set_descriptors = descriptors->NumberOfSetDescriptors();
+  int new_last_added = descriptors->Append(desc, witness, set_descriptors);
+  descriptors->SetLastAdded(new_last_added);
+}
+
 
 Object* Map::GetBackPointer() {
   Object* object = READ_FIELD(this, kInstanceDescriptorsOrBackPointerOffset);