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

test/mjsunit/harmony/to-string.js

Index: test/mjsunit/harmony/to-string.js
diff --git a/test/mjsunit/harmony/to-string.js b/test/mjsunit/harmony/to-string.js
index 4f03c466ea4ceea8c559f406f380b4e2f99e287c..ccc634f03e4f77688135409efca493826c783c9f 100644
--- a/test/mjsunit/harmony/to-string.js
+++ b/test/mjsunit/harmony/to-string.js
@@ -48,3 +48,7 @@ var d = {
 };
 assertEquals("string", %ToString(d));
 assertEquals("string", %_ToString(d));
+
+var e = new Date(0);
+assertEquals("Thu Jan 01 1970 01:00:00 GMT+0100 (CET)", %ToString(e));
+assertEquals("Thu Jan 01 1970 01:00:00 GMT+0100 (CET)", %_ToString(e));