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

src/builtins/builtins-symbol.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/builtins/builtins.h"
 #include "src/builtins/builtins-utils.h"
 
 namespace v8 {
 namespace internal {
 
@@ skipping 14 matching lines @@
 }
 
 // ES6 section 19.4.1.1 Symbol ( [ description ] ) for the [[Construct]] case.
 BUILTIN(SymbolConstructor_ConstructStub) {
   HandleScope scope(isolate);
   THROW_NEW_ERROR_RETURN_FAILURE(
       isolate, NewTypeError(MessageTemplate::kNotConstructor,
                             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) {
   typedef compiler::Node Node;
   CodeStubAssembler assembler(state);
 
   Node* receiver = assembler.Parameter(0);
   Node* context = assembler.Parameter(4);
 
   Node* result =
@@ skipping 27 matching lines @@
   Node* receiver = assembler.Parameter(0);
   Node* context = assembler.Parameter(3);
 
   Node* result = assembler.ToThisValue(
       context, receiver, PrimitiveType::kSymbol, "Symbol.prototype.valueOf");
   assembler.Return(result);
 }
 
 }  // namespace internal
 }  // namespace v8