[es6] Implement spec compliant ToPrimitive in the runtime.

src/symbol.js

 // Copyright 2013 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.
 
 // Expects following symbols to be set in the bootstrapper during genesis:
 // - symbolHasInstance
 // - symbolIsConcatSpreadable
 // - symbolIsRegExp
 // - symbolIterator
+// - symbolToPrimitive
 // - symbolToStringTag
 // - symbolUnscopables
 
 var $symbolToString;
 
 (function(global, utils) {
 
 "use strict";
 
 %CheckIsBootstrapping();
@@ skipping 13 matching lines @@
 
 // -------------------------------------------------------------------
 
 function SymbolConstructor(x) {
   if (%_IsConstructCall()) throw MakeTypeError(kNotConstructor, "Symbol");
   // NOTE: Passing in a Symbol value will throw on ToString().
   return %CreateSymbol(IS_UNDEFINED(x) ? x : ToString(x));
 }
 
 
+// 19.4.3.4 Symbol.prototype [ @@toPrimitive ] ( hint )
+function SymbolToPrimitive(hint) {
+  if (!(IS_SYMBOL(this) || IS_SYMBOL_WRAPPER(this))) {
+    throw MakeTypeError(kIncompatibleMethodReceiver,
+                        "Symbol.prototype [ @@toPrimitive ]", this);
+  }
+  return %_ValueOf(this);
+}
+
+
 function SymbolToString() {
   if (!(IS_SYMBOL(this) || IS_SYMBOL_WRAPPER(this))) {
     throw MakeTypeError(kIncompatibleMethodReceiver,
                         "Symbol.prototype.toString", this);
   }
   var description = %SymbolDescription(%_ValueOf(this));
   return "Symbol(" + (IS_UNDEFINED(description) ? "" : description) + ")";
 }
 
 
@@ skipping 37 matching lines @@
 
 %SetCode(GlobalSymbol, SymbolConstructor);
 %FunctionSetPrototype(GlobalSymbol, new GlobalObject());
 
 utils.InstallConstants(GlobalSymbol, [
   // TODO(rossberg): expose when implemented.
   // "hasInstance", symbolHasInstance,
   // "isConcatSpreadable", symbolIsConcatSpreadable,
   // "isRegExp", symbolIsRegExp,
   "iterator", symbolIterator,
+  "toPrimitive", symbolToPrimitive,
   // TODO(dslomov, caitp): Currently defined in harmony-tostring.js ---
   // Move here when shipping
   // "toStringTag", symbolToStringTag,
   "unscopables", symbolUnscopables
 ]);
 
 utils.InstallFunctions(GlobalSymbol, DONT_ENUM, [
   "for", SymbolFor,
   "keyFor", SymbolKeyFor
 ]);
 
 %AddNamedProperty(
     GlobalSymbol.prototype, "constructor", GlobalSymbol, DONT_ENUM);
+utils.SetFunctionName(SymbolToPrimitive, symbolToPrimitive);
+%AddNamedProperty(
+    GlobalSymbol.prototype, symbolToPrimitive, SymbolToPrimitive,
+    DONT_ENUM | READ_ONLY);
 %AddNamedProperty(
     GlobalSymbol.prototype, symbolToStringTag, "Symbol", DONT_ENUM | READ_ONLY);
 
 utils.InstallFunctions(GlobalSymbol.prototype, DONT_ENUM, [
   "toString", SymbolToString,
   "valueOf", SymbolValueOf
 ]);
 
 utils.InstallFunctions(GlobalObject, DONT_ENUM, [
   "getOwnPropertySymbols", ObjectGetOwnPropertySymbols
 ]);
 
 $symbolToString = SymbolToString;
 
 })