Introduce extras export object.

src/contexts.h

Index: src/contexts.h
diff --git a/src/contexts.h b/src/contexts.h
index f120788609f2d748ed0015684cc0f91f0900d899..94c170c5e55ab28675b30629c97b1d714bc4a567 100644
--- a/src/contexts.h
+++ b/src/contexts.h
@@ -187,7 +187,8 @@ enum BindingFlags {
   V(SET_ITERATOR_MAP_INDEX, Map, set_iterator_map)                             \
   V(ARRAY_VALUES_ITERATOR_INDEX, JSFunction, array_values_iterator)            \
   V(SCRIPT_CONTEXT_TABLE_INDEX, ScriptContextTable, script_context_table)      \
-  V(BUILTIN_EXPORTS_OBJECT_INDEX, Object, builtin_exports_object)
+  V(BUILTIN_EXPORTS_OBJECT_INDEX, Object, builtin_exports_object)              \
+  V(EXTRAS_EXPORTS_OBJECT_INDEX, JSObject, extras_exports_object)
 
 
 // A table of all script contexts. Every loaded top-level script with top-level
@@ -426,6 +427,7 @@ class Context: public FixedArray {
     MAP_CACHE_INDEX,
     TO_LENGTH_FUN_INDEX,
     BUILTIN_EXPORTS_OBJECT_INDEX,
+    EXTRAS_EXPORTS_OBJECT_INDEX,
 
     // Properties from here are treated as weak references by the full GC.
     // Scavenge treats them as strong references.