Replace AddProperty by AddNamedProperty to speed up the common case

src/string-iterator.js

Index: src/string-iterator.js
diff --git a/src/string-iterator.js b/src/string-iterator.js
index 7a2309de705c4a19fa86e8d6927641c043a939d2..7222885a56f465931980d646a277ea9c69cc1abf 100644
--- a/src/string-iterator.js
+++ b/src/string-iterator.js
@@ -84,8 +84,8 @@ function SetUpStringIterator() {
     'next', StringIteratorNext
   ));
   %FunctionSetName(StringIteratorIterator, '[Symbol.iterator]');
-  %AddProperty(StringIterator.prototype, symbolIterator, StringIteratorIterator,
-      DONT_ENUM);
+  %AddNamedProperty(StringIterator.prototype, symbolIterator,
+                    StringIteratorIterator, DONT_ENUM);
 }
 SetUpStringIterator();
 
@@ -100,7 +100,7 @@ function ExtendStringPrototypeWithIterator() {
   %CheckIsBootstrapping();
 
   %FunctionSetName(StringPrototypeIterator, '[Symbol.iterator]');
-  %AddProperty($String.prototype, symbolIterator, StringPrototypeIterator,
-      DONT_ENUM);
+  %AddNamedProperty($String.prototype, symbolIterator,
+                    StringPrototypeIterator, DONT_ENUM);
 }
 ExtendStringPrototypeWithIterator();