v8::Private::ForApi should be context-independent.

src/builtins/builtins-symbol.cc

Index: src/builtins/builtins-symbol.cc
diff --git a/src/builtins/builtins-symbol.cc b/src/builtins/builtins-symbol.cc
index 209bd58f6faaa3ac09528829be9b60af492a7eca..b422d8f321fefbfb66f055958345689c2ba7f3c1 100644
--- a/src/builtins/builtins-symbol.cc
+++ b/src/builtins/builtins-symbol.cc
@@ -32,6 +32,38 @@ BUILTIN(SymbolConstructor_ConstructStub) {
                             isolate->factory()->Symbol_string()));
 }
 
+// ES6 section 19.4.2.1 Symbol.for.
+BUILTIN(SymbolFor) {
+  HandleScope scope(isolate);
+  Handle<Object> key_obj = args.atOrUndefined(isolate, 1);
+  Handle<String> key;
+  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, key,
+                                     Object::ToString(isolate, key_obj));
+  return *isolate->SymbolFor(Heap::kPublicSymbolTableRootIndex, key, false);
+}
+
+// ES6 section 19.4.2.5 Symbol.keyFor.
+BUILTIN(SymbolKeyFor) {
+  HandleScope scope(isolate);
+  Handle<Object> obj = args.atOrUndefined(isolate, 1);
+  if (!obj->IsSymbol()) {
+    THROW_NEW_ERROR_RETURN_FAILURE(
+        isolate, NewTypeError(MessageTemplate::kSymbolKeyFor, obj));
+  }
+  Handle<Symbol> symbol = Handle<Symbol>::cast(obj);
+  DisallowHeapAllocation no_gc;
+  Object* result;
+  if (symbol->is_public()) {
+    result = symbol->name();
+    DCHECK(result->IsString());
+  } else {
+    result = isolate->heap()->undefined_value();
+  }
+  DCHECK_EQ(isolate->heap()->public_symbol_table()->SlowReverseLookup(*symbol),
+            result);
+  return result;
+}
+
 // ES6 section 19.4.3.4 Symbol.prototype [ @@toPrimitive ] ( hint )
 void Builtins::Generate_SymbolPrototypeToPrimitive(
     compiler::CodeAssemblerState* state) {