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 c7096724ace127053e80327f116de8c3d34ac646..0957adcff5cc7555b24c7197a07ea8ce618c4d64 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -181,7 +181,10 @@ 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_data_undefined:         ["Symbol wrapper object has no symbol data"],
+  symbol_value_expected:         ["Symbol wrapper constructor called with a non-Symbol value argument"],
+  symbol_to_string:              ["Cannot convert Symbol values to strings"],
+  symbol_to_primitive:           ["Cannot convert Symbol wrapper objects to primitive values"],
   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"]