Add support for per-isolate private symbols

src/symbol.js

Index: src/symbol.js
diff --git a/src/symbol.js b/src/symbol.js
index 49f909694c19f3ee44c60a39def7fbbd0f39c6a5..550ebe1f7a8d6cf360c9233335091fc170b19f9f 100644
--- a/src/symbol.js
+++ b/src/symbol.js
@@ -63,30 +63,19 @@ function SymbolValueOf() {
 }
 
 
-function GetSymbolRegistry() {
-  var registry = %SymbolRegistry();
-  if (!('internal' in registry)) {
-    registry.internal = {__proto__: null};
-    registry.for = {__proto__: null};
-    registry.keyFor = {__proto__: null};
-  }
-  return registry;
-}
-
-
 function InternalSymbol(key) {
-  var registry = GetSymbolRegistry();
-  if (!(key in registry.internal)) {
-    registry.internal[key] = %CreateSymbol(key);
+  var internal_registry = %SymbolRegistry().for_intern;
+  if (IS_UNDEFINED(internal_registry[key])) {
+    internal_registry[key] = %CreateSymbol(key);
   }
-  return registry.internal[key];
+  return internal_registry[key];
 }
 
 
 function SymbolFor(key) {
   key = TO_STRING_INLINE(key);
-  var registry = GetSymbolRegistry();
-  if (!(key in registry.for)) {
+  var registry = %SymbolRegistry();
+  if (IS_UNDEFINED(registry.for[key])) {
     var symbol = %CreateSymbol(key);
     registry.for[key] = symbol;
     registry.keyFor[symbol] = key;
@@ -96,10 +85,8 @@ function SymbolFor(key) {
 
 
 function SymbolKeyFor(symbol) {
-  if (!IS_SYMBOL(symbol)) {
-    throw MakeTypeError("not_a_symbol", [symbol]);
-  }
-  return GetSymbolRegistry().keyFor[symbol];
+  if (!IS_SYMBOL(symbol)) throw MakeTypeError("not_a_symbol", [symbol]);
+  return %SymbolRegistry().keyFor[symbol];
 }