[es6] Better support for built-ins subclassing.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index d16b0e2b72091fcda20e6a6c9a04f205f67b6410..134a92cd029b2782788d08be1015c4c6d8f1b081 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2132,8 +2132,10 @@ void WeakCell::clear_next(Heap* heap) {
 bool WeakCell::next_cleared() { return next()->IsTheHole(); }
 
 
-int JSObject::GetHeaderSize() {
-  InstanceType type = map()->instance_type();
+int JSObject::GetHeaderSize() { return GetHeaderSize(map()->instance_type()); }
+
+
+int JSObject::GetHeaderSize(InstanceType type) {
   // Check for the most common kind of JavaScript object before
   // falling into the generic switch. This speeds up the internal
   // field operations considerably on average.
@@ -2190,15 +2192,18 @@ int JSObject::GetHeaderSize() {
 }
 
 
-int JSObject::GetInternalFieldCount() {
-  DCHECK(1 << kPointerSizeLog2 == kPointerSize);
-  // Make sure to adjust for the number of in-object properties. These
-  // properties do contribute to the size, but are not internal fields.
-  return ((Size() - GetHeaderSize()) >> kPointerSizeLog2) -
-         map()->GetInObjectProperties();
+int JSObject::GetInternalFieldCount(Map* map) {
+  int instance_size = map->instance_size();
+  if (instance_size == kVariableSizeSentinel) return 0;
+  InstanceType instance_type = map->instance_type();
+  return ((instance_size - GetHeaderSize(instance_type)) >> kPointerSizeLog2) -
+         map->GetInObjectProperties();
 }
 
 
+int JSObject::GetInternalFieldCount() { return GetInternalFieldCount(map()); }
+
+
 int JSObject::GetInternalFieldOffset(int index) {
   DCHECK(index < GetInternalFieldCount() && index >= 0);
   return GetHeaderSize() + (kPointerSize * index);
@@ -5622,6 +5627,12 @@ void Map::SetConstructor(Object* constructor, WriteBarrierMode mode) {
 }
 
 
+Handle<Map> Map::CopyInitialMap(Handle<Map> map) {
+  return CopyInitialMap(map, map->instance_size(), map->GetInObjectProperties(),
+                        map->unused_property_fields());
+}
+
+
 ACCESSORS(JSFunction, shared, SharedFunctionInfo, kSharedFunctionInfoOffset)
 ACCESSORS(JSFunction, literals_or_bindings, FixedArray, kLiteralsOffset)
 ACCESSORS(JSFunction, next_function_link, Object, kNextFunctionLinkOffset)