[es6] Implement spec compliant ToPrimitive in the runtime.

src/symbol.js

Index: src/symbol.js
diff --git a/src/symbol.js b/src/symbol.js
index 8cac2c56a2f69cfa8397c6c13bb0d07ef9f33f1f..3f6e281777086cb9317071a730767db99c9db157 100644
--- a/src/symbol.js
+++ b/src/symbol.js
@@ -7,6 +7,7 @@
 // - symbolIsConcatSpreadable
 // - symbolIsRegExp
 // - symbolIterator
+// - symbolToPrimitive
 // - symbolToStringTag
 // - symbolUnscopables
 
@@ -40,6 +41,16 @@ function SymbolConstructor(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,
@@ -97,6 +108,7 @@ utils.InstallConstants(GlobalSymbol, [
   // "isConcatSpreadable", symbolIsConcatSpreadable,
   // "isRegExp", symbolIsRegExp,
   "iterator", symbolIterator,
+  "toPrimitive", symbolToPrimitive,
   // TODO(dslomov, caitp): Currently defined in harmony-tostring.js ---
   // Move here when shipping
   // "toStringTag", symbolToStringTag,
@@ -110,6 +122,10 @@ utils.InstallFunctions(GlobalSymbol, DONT_ENUM, [
 
 %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);