!fixup fix native context constant for v6.x

Author: Matheus Marchini

src/llv8-constants.cc

@@ -239,8 +239,13 @@ void Context::Load() {
       LoadConstant("class_Context__closure_index__int", "context_idx_closure");
   kPreviousIndex =
       LoadConstant("class_Context__previous_index__int", "context_idx_prev");
+  // TODO (mmarchini) change LoadConstant to accept variable arguments, a list
+  // of constants or a fallback list).
   kNativeIndex =
       LoadConstant("class_Context__native_index__int", "context_idx_native");
+  if (kNativeIndex == -1) {
+    kNativeIndex = LoadConstant("class_Context__native_context_index__int");
+  }
   kEmbedderDataIndex = LoadConstant("context_idx_embedder_data", (int)5);
 
   kMinContextSlots = LoadConstant("class_Context__min_context_slots__int",