ES6 symbols: turn symbols into a proper primitive type

src/contexts.h

Index: src/contexts.h
diff --git a/src/contexts.h b/src/contexts.h
index cd7ed6adc5ba165b4302e87c23aa9da2ba0e8f11..a0ba2f7eb5f577efed6e7da90a4f63dac0977d27 100644
--- a/src/contexts.h
+++ b/src/contexts.h
@@ -103,6 +103,7 @@ enum BindingFlags {
   V(NUMBER_FUNCTION_INDEX, JSFunction, number_function) \
   V(STRING_FUNCTION_INDEX, JSFunction, string_function) \
   V(STRING_FUNCTION_PROTOTYPE_MAP_INDEX, Map, string_function_prototype_map) \
+  V(SYMBOL_FUNCTION_INDEX, JSFunction, symbol_function) \
   V(OBJECT_FUNCTION_INDEX, JSFunction, object_function) \
   V(INTERNAL_ARRAY_FUNCTION_INDEX, JSFunction, internal_array_function) \
   V(ARRAY_FUNCTION_INDEX, JSFunction, array_function) \
@@ -156,7 +157,6 @@ enum BindingFlags {
   V(ALLOW_CODE_GEN_FROM_STRINGS_INDEX, Object, allow_code_gen_from_strings) \
   V(ERROR_MESSAGE_FOR_CODE_GEN_FROM_STRINGS_INDEX, Object, \
     error_message_for_code_gen_from_strings) \
-  V(SYMBOL_DELEGATE_INDEX, JSObject, symbol_delegate) \
   V(TO_COMPLETE_PROPERTY_DESCRIPTOR_INDEX, JSFunction, \
     to_complete_property_descriptor) \
   V(DERIVED_HAS_TRAP_INDEX, JSFunction, derived_has_trap) \
@@ -251,6 +251,7 @@ class Context: public FixedArray {
     NUMBER_FUNCTION_INDEX,
     STRING_FUNCTION_INDEX,
     STRING_FUNCTION_PROTOTYPE_MAP_INDEX,
+    SYMBOL_FUNCTION_INDEX,
     OBJECT_FUNCTION_INDEX,
     INTERNAL_ARRAY_FUNCTION_INDEX,
     ARRAY_FUNCTION_INDEX,
@@ -287,7 +288,6 @@ class Context: public FixedArray {
     EMBEDDER_DATA_INDEX,
     ALLOW_CODE_GEN_FROM_STRINGS_INDEX,
     ERROR_MESSAGE_FOR_CODE_GEN_FROM_STRINGS_INDEX,
-    SYMBOL_DELEGATE_INDEX,
     TO_COMPLETE_PROPERTY_DESCRIPTOR_INDEX,
     DERIVED_HAS_TRAP_INDEX,
     DERIVED_GET_TRAP_INDEX,