[es6] Implement Date.prototype[@@toPrimitive] as C++ builtin.

src/builtins.h

Index: src/builtins.h
diff --git a/src/builtins.h b/src/builtins.h
index 5c78877803d1d8c17959872d51032f8b532ee7d5..694273afd99fa99064374f49f4030db1e045f820 100644
--- a/src/builtins.h
+++ b/src/builtins.h
@@ -57,6 +57,8 @@ enum BuiltinExtraArguments {
   V(ArraySplice, NO_EXTRA_ARGUMENTS)                         \
   V(ArrayConcat, NO_EXTRA_ARGUMENTS)                         \
                                                              \
+  V(DateToPrimitive, NO_EXTRA_ARGUMENTS)                     \
+                                                             \
   V(HandleApiCall, NEEDS_CALLED_FUNCTION)                    \
   V(HandleApiCallConstruct, NEEDS_CALLED_FUNCTION)           \
   V(HandleApiCallAsFunction, NO_EXTRA_ARGUMENTS)             \