Replace AddProperty by AddNamedProperty to speed up the common case

src/symbol.js

Index: src/symbol.js
diff --git a/src/symbol.js b/src/symbol.js
index c3e2aec0bce490349c0fd1ed14cdb0691a964b33..0ac3a125a0998ec0ab2fe6aa1385aa0226da7f82 100644
--- a/src/symbol.js
+++ b/src/symbol.js
@@ -113,7 +113,7 @@ function SetUpSymbol() {
     "keyFor", SymbolKeyFor
   ));
 
-  %AddProperty($Symbol.prototype, "constructor", $Symbol, DONT_ENUM);
+  %AddNamedProperty($Symbol.prototype, "constructor", $Symbol, DONT_ENUM);
   InstallFunctions($Symbol.prototype, DONT_ENUM, $Array(
     "toString", SymbolToString,
     "valueOf", SymbolValueOf