[turbofan] First steps towards optimizing for-in loops.

src/compiler/js-graph.cc

Index: src/compiler/js-graph.cc
diff --git a/src/compiler/js-graph.cc b/src/compiler/js-graph.cc
index 4d83c86eebc39e8e96569defe8b447057ccc1e0a..35984b8357b08a6aa5f2c88958605922aee06d43 100644
--- a/src/compiler/js-graph.cc
+++ b/src/compiler/js-graph.cc
@@ -183,6 +183,11 @@ Node* JSGraph::ExternalConstant(ExternalReference reference) {
 }
 
 
+Node* JSGraph::ExternalConstant(Runtime::FunctionId function_id) {
+  return ExternalConstant(ExternalReference(function_id, isolate()));
+}
+
+
 Node* JSGraph::EmptyFrameState() {
   Node* empty_frame_state = cached_nodes_[kEmptyFrameState];
   if (!empty_frame_state || empty_frame_state->IsDead()) {