Upgrade Symbol implementation to match current ES6 behavior.

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 41e1d9781df37b0ff2f2757392641d32009fe175..09ef7a5539cf45d737f5d81967b7e6e7ae89c8e7 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -184,7 +184,8 @@ var kMessages = {
   cant_prevent_ext_external_array_elements: ["Cannot prevent extension of an object with external array elements"],
   redef_external_array_element:  ["Cannot redefine a property of an object with external array elements"],
   harmony_const_assign:          ["Assignment to constant variable."],
-  symbol_to_string:              ["Conversion from symbol to string"],
+  symbol_to_string:              ["Cannot convert a Symbol value to a string"],
+  symbol_to_primitive:           ["Cannot convert a Symbol wrapper object to a primitive value"],
   invalid_module_path:           ["Module does not export '", "%0", "', or export is not itself a module"],
   module_type_error:             ["Module '", "%0", "' used improperly"],
   module_export_undefined:       ["Export '", "%0", "' is not defined in module"]