Fastpath some spread-call desugaring.

src/runtime/runtime-array.cc

Index: src/runtime/runtime-array.cc
diff --git a/src/runtime/runtime-array.cc b/src/runtime/runtime-array.cc
index ef177fec4c193e0fa9e83f63d680353924d68e7a..7d6ffc39553d09e4538303efb94bfcdb6259d330 100644
--- a/src/runtime/runtime-array.cc
+++ b/src/runtime/runtime-array.cc
@@ -628,5 +628,34 @@ RUNTIME_FUNCTION(Runtime_ArrayIndexOf) {
   return Smi::FromInt(-1);
 }
 
+RUNTIME_FUNCTION(Runtime_SpreadIterablePrepare) {
+  HandleScope scope(isolate);
+  DCHECK_EQ(1, args.length());
+  CONVERT_ARG_HANDLE_CHECKED(Object, spread, 0);
+
+  if (spread->IsJSArray()) {
+    // Check that the spread arg has fast elements
+    Handle<JSArray> spread_array = Handle<JSArray>::cast(spread);
+    ElementsKind array_kind = spread_array->GetElementsKind();
+
+    if (IsFastElementsKind(array_kind)) {
+      // Check that the iterator acts as expected.
+      if (isolate->IsArrayIteratorLookupChainIntact()) {
+        return *spread;
+      }
+    }
+  }
+
+  Handle<JSFunction> spread_iterable_function = isolate->spread_iterable();
+
+  Handle<Object> spreaded;
+  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
+      isolate, spreaded,
+      Execution::Call(isolate, spread_iterable_function,
+                      isolate->factory()->undefined_value(), 1, &spread));
+
+  return *spreaded;
+}
+
 }  // namespace internal
 }  // namespace v8