Replace AddProperty by AddNamedProperty to speed up the common case

src/date.js

Index: src/date.js
diff --git a/src/date.js b/src/date.js
index c4c7f6e3830524be94ab8da5dd894626aebcf5ae..87c87bfda6023355e47a1908dd081701640e124e 100644
--- a/src/date.js
+++ b/src/date.js
@@ -763,7 +763,7 @@ function SetUpDate() {
   ));
 
   // Set up non-enumerable constructor property of the Date prototype object.
-  %AddProperty($Date.prototype, "constructor", $Date, DONT_ENUM);
+  %AddNamedProperty($Date.prototype, "constructor", $Date, DONT_ENUM);
 
   // Set up non-enumerable functions of the Date prototype object and
   // set their names.