[builtins] Migrate Object.keys to CodeStubAssembler builtin.

src/keys.cc

Index: src/keys.cc
diff --git a/src/keys.cc b/src/keys.cc
index da26263fc49c2eac2a4e6435770d604c26a7530f..4af11f85c44e773ffaf436061153f3ea1b8c0489 100644
--- a/src/keys.cc
+++ b/src/keys.cc
@@ -47,7 +47,8 @@ Handle<FixedArray> KeyAccumulator::GetKeys(GetKeysConversion convert) {
     return isolate_->factory()->empty_fixed_array();
   }
   if (mode_ == KeyCollectionMode::kOwnOnly &&
-      keys_->map() == isolate_->heap()->fixed_array_map()) {
+      (keys_->map() == isolate_->heap()->fixed_array_map() ||
+       keys_->map() == isolate_->heap()->fixed_cow_array_map())) {
     return Handle<FixedArray>::cast(keys_);
   }
   USE(ContainsOnlyValidKeys);
@@ -263,9 +264,9 @@ void FastKeyAccumulator::Prepare() {
 }
 
 namespace {
-static Handle<FixedArray> ReduceFixedArrayTo(Isolate* isolate,
-                                             Handle<FixedArray> array,
-                                             int length) {
+
+Handle<FixedArray> ReduceFixedArrayTo(Isolate* isolate,
+                                      Handle<FixedArray> array, int length) {
   DCHECK_LE(length, array->length());
   if (array->length() == length) return array;
   return isolate->factory()->CopyFixedArrayUpTo(array, length);
@@ -315,33 +316,39 @@ Handle<FixedArray> GetFastEnumPropertyKeys(Isolate* isolate,
 
   isolate->counters()->enum_cache_misses()->Increment();
 
-  Handle<FixedArray> storage =
-      isolate->factory()->NewFixedArray(own_property_count);
-  Handle<FixedArray> indices =
-      isolate->factory()->NewFixedArray(own_property_count);
-
-  int size = map->NumberOfOwnDescriptors();
-  int index = 0;
-
-  for (int i = 0; i < size; i++) {
-    PropertyDetails details = descs->GetDetails(i);
-    if (details.IsDontEnum()) continue;
-    Object* key = descs->GetKey(i);
-    if (key->IsSymbol()) continue;
-    storage->set(index, key);
-    if (!indices.is_null()) {
-      if (details.location() == kField) {
-        DCHECK_EQ(kData, details.kind());
-        FieldIndex field_index = FieldIndex::ForDescriptor(*map, i);
-        int load_by_field_index = field_index.GetLoadByFieldIndex();
-        indices->set(index, Smi::FromInt(load_by_field_index));
-      } else {
-        indices = Handle<FixedArray>();
+  Handle<FixedArray> storage = isolate->factory()->empty_fixed_array();
+  Handle<FixedArray> indices = isolate->factory()->empty_fixed_array();
+  if (own_property_count != 0) {
+    storage = isolate->factory()->NewFixedArray(own_property_count);
+    indices = isolate->factory()->NewFixedArray(own_property_count);
+
+    int size = map->NumberOfOwnDescriptors();
+    int index = 0;
+
+    for (int i = 0; i < size; i++) {
+      PropertyDetails details = descs->GetDetails(i);
+      if (details.IsDontEnum()) continue;
+      Object* key = descs->GetKey(i);
+      if (key->IsSymbol()) continue;
+      storage->set(index, key);
+      if (!indices.is_null()) {
+        if (details.location() == kField) {
+          DCHECK_EQ(kData, details.kind());
+          FieldIndex field_index = FieldIndex::ForDescriptor(*map, i);
+          int load_by_field_index = field_index.GetLoadByFieldIndex();
+          indices->set(index, Smi::FromInt(load_by_field_index));
+        } else {
+          indices = Handle<FixedArray>();
+        }
       }
+      index++;
     }
-    index++;
+    DCHECK_EQ(index, storage->length());
+
+    // Mark the {storage} as copy-on-write, so we can use this
+    // as JSArray elements backing store directly w/o copying.
+    storage->set_map(isolate->heap()->fixed_cow_array_map());
   }
-  DCHECK(index == storage->length());
 
   DescriptorArray::SetEnumCache(descs, isolate, storage, indices);
   if (cache_enum_length) {
@@ -442,10 +449,7 @@ FastKeyAccumulator::GetOwnKeysWithUninitializedEnumCache() {
   }
   // We have no elements but possibly enumerable property keys, hence we can
   // directly initialize the enum cache.
-  Handle<FixedArray> keys = GetFastEnumPropertyKeys(isolate_, object);
-  if (is_for_in_) return keys;
-  // Do not leak the enum cache as it might end up as an elements backing store.
-  return isolate_->factory()->CopyFixedArray(keys);
+  return GetFastEnumPropertyKeys(isolate_, object);
 }
 
 MaybeHandle<FixedArray> FastKeyAccumulator::GetKeysSlow(