Implement ES6 symbol registry and predefined symbols

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index e6365a441e32c732ab2a0aad9a455a2523fdebbe..6abaa4e8e3afbc5f5c6e5135c68295fd6cde3a66 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -103,6 +103,7 @@ var kMessages = {
   invalid_argument:              ["invalid_argument"],
   data_view_not_array_buffer:    ["First argument to DataView constructor must be an ArrayBuffer"],
   constructor_not_function:      ["Constructor ", "%0", " requires 'new'"],
+  not_a_symbol:                  ["%0", " is not a symbol"],
   not_a_promise:                 ["%0", " is not a promise"],
   resolver_not_a_function:       ["Promise resolver ", "%0", " is not a function"],
   promise_cyclic:                ["Chaining cycle detected for promise ", "%0"],