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..b4a85af3dd7d287605457eb088c6dac1a942079b 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -181,7 +181,9 @@ 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 Symbol values to strings"],

[arv (Not doing code reviews), 2014/02/01 01:33:10]

"value to string" maybe?

+  symbol_to_value:               ["Cannot call valueOf() on a Symbol value."],
+  symbol_to_primitive:           ["Cannot convert Symbol wrapper objects to primitive values"],

[arv (Not doing code reviews), 2014/02/01 01:33:10]

same?

   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"]