For-of calls [Symbol.iterator]() on RHS to get iterator

src/contexts.h

Index: src/contexts.h
diff --git a/src/contexts.h b/src/contexts.h
index d9541b0be2e1f1a761b50052a077f26d2a7f4841..5fad02d23cad25ddd56ef56a0d5c5ed05a9938e1 100644
--- a/src/contexts.h
+++ b/src/contexts.h
@@ -186,7 +186,8 @@ enum BindingFlags {
     generator_object_prototype_map) \
   V(ITERATOR_RESULT_MAP_INDEX, Map, iterator_result_map) \
   V(MAP_ITERATOR_MAP_INDEX, Map, map_iterator_map) \
-  V(SET_ITERATOR_MAP_INDEX, Map, set_iterator_map)
+  V(SET_ITERATOR_MAP_INDEX, Map, set_iterator_map) \
+  V(ITERATOR_SYMBOL_INDEX, Symbol, iterator_symbol)
 
 // JSFunctions are pairs (context, function code), sometimes also called
 // closures. A Context object is used to represent function contexts and
@@ -358,6 +359,7 @@ class Context: public FixedArray {
     ITERATOR_RESULT_MAP_INDEX,
     MAP_ITERATOR_MAP_INDEX,
     SET_ITERATOR_MAP_INDEX,
+    ITERATOR_SYMBOL_INDEX,
 
     // Properties from here are treated as weak references by the full GC.
     // Scavenge treats them as strong references.