[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 3bda179712b010834977212ceffa36772927a94a..4918ff7fd5b73ff592abdca4a72e40344b9587ea 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2125,8 +2125,10 @@ 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.
@@ -2183,10 +2185,64 @@ int JSObject::GetHeaderSize() {
 }
 
 
+int JSObject::GetHeaderWithInternalFieldsSize(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.
+  if (type == JS_OBJECT_TYPE) return JSObject::kHeaderSize;
+  switch (type) {
+    case JS_GENERATOR_OBJECT_TYPE:
+      return JSGeneratorObject::kSize;
+    case JS_MODULE_TYPE:
+      return JSModule::kSize;
+    case JS_GLOBAL_PROXY_TYPE:
+      return JSGlobalProxy::kSize;
+    case JS_GLOBAL_OBJECT_TYPE:
+      return JSGlobalObject::kSize;
+    case JS_BUILTINS_OBJECT_TYPE:
+      return JSBuiltinsObject::kSize;
+    case JS_FUNCTION_TYPE:
+      return JSFunction::kSize;
+    case JS_VALUE_TYPE:
+      return JSValue::kSize;
+    case JS_DATE_TYPE:
+      return JSDate::kSize;
+    case JS_ARRAY_TYPE:
+      return JSArray::kSize;
+    case JS_ARRAY_BUFFER_TYPE:
+      return JSArrayBuffer::kSizeWithInternalFields;
+    case JS_TYPED_ARRAY_TYPE:
+      return JSTypedArray::kSizeWithInternalFields;
+    case JS_DATA_VIEW_TYPE:
+      return JSDataView::kSizeWithInternalFields;
+    case JS_SET_TYPE:
+      return JSSet::kSize;
+    case JS_MAP_TYPE:
+      return JSMap::kSize;
+    case JS_SET_ITERATOR_TYPE:
+      return JSSetIterator::kSize;
+    case JS_MAP_ITERATOR_TYPE:
+      return JSMapIterator::kSize;
+    case JS_ITERATOR_RESULT_TYPE:
+      return JSIteratorResult::kSize;
+    case JS_WEAK_MAP_TYPE:
+      return JSWeakMap::kSize;
+    case JS_WEAK_SET_TYPE:
+      return JSWeakSet::kSize;
+    case JS_REGEXP_TYPE:
+      return JSRegExp::kSize;
+    case JS_CONTEXT_EXTENSION_OBJECT_TYPE:
+      return JSObject::kHeaderSize;
+    case JS_MESSAGE_OBJECT_TYPE:
+      return JSMessageObject::kSize;
+    default:
+      UNREACHABLE();
+      return 0;
+  }
+}
+
+
 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();
 }