Replace AddProperty by AddNamedProperty to speed up the common case

src/string.js

Index: src/string.js
diff --git a/src/string.js b/src/string.js
index d949c644337f8cb67555392c2b02ec862cde0731..9719df4c288f2a53af851574fda85180fa0b27e8 100644
--- a/src/string.js
+++ b/src/string.js
@@ -915,7 +915,7 @@ function SetUpString() {
   %FunctionSetPrototype($String, new $String());
 
   // Set up the constructor property on the String prototype object.
-  %AddProperty($String.prototype, "constructor", $String, DONT_ENUM);
+  %AddNamedProperty($String.prototype, "constructor", $String, DONT_ENUM);
 
   // Set up the non-enumerable functions on the String object.
   InstallFunctions($String, DONT_ENUM, $Array(