[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..103ba89d1dca1787488eedc9684363c35b459e42 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(e.toString(), %ToName(e));
+assertEquals(e.toString(), %_ToName(e));