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..48c88c33c75c6f4569d97a682368e007f7a9d033 100644
--- a/src/symbol.js
+++ b/src/symbol.js
@@ -65,10 +65,13 @@ function SymbolValueOf() {
 
 function GetSymbolRegistry() {
   var registry = %SymbolRegistry();
-  if (!('internal' in registry)) {
-    registry.internal = {__proto__: null};
+  if (!('for' in registry)) {
     registry.for = {__proto__: null};
+    registry.for_intern = {__proto__: null};
     registry.keyFor = {__proto__: null};
+    // Other members are added and used in runtime and API functions:
+    // registry.private_intern
+    // registry.private_api
   }
   return registry;
 }
@@ -76,10 +79,10 @@ function GetSymbolRegistry() {
 
 function InternalSymbol(key) {
   var registry = GetSymbolRegistry();
-  if (!(key in registry.internal)) {
-    registry.internal[key] = %CreateSymbol(key);
+  if (!(key in registry.for_intern)) {
+    registry.for_intern[key] = %CreateSymbol(key);
   }
-  return registry.internal[key];
+  return registry.for_intern[key];
 }