ES6 symbols: turn symbols into a proper primitive type

src/symbol.js

Index: src/symbol.js
diff --git a/src/symbol.js b/src/symbol.js
index b7f9dc9496aac589e89e2d2761ebe6431a36008e..b67e00b89548e34d4ec32d60ad1043356b567624 100644
--- a/src/symbol.js
+++ b/src/symbol.js
@@ -27,13 +27,44 @@
 
 "use strict";
 
-var $Symbol = function() { return %CreateSymbol() }
-global.Symbol = $Symbol
+var $Symbol = global.Symbol;
 
-// Symbols only have a toString method and no prototype.
-var SymbolDelegate = {
-  __proto__: null,
-  toString: $Object.prototype.toString
+function SymbolConstructor(x) {
+  var value = IS_SYMBOL(x) ? x : %CreateSymbol();
+  if (%_IsConstructCall()) {
+    %_SetValueOf(this, value);
+  } else {
+    return value;
+  }
 }
 
-$Object.freeze(SymbolDelegate)
+function SymbolToString() {
+  throw MakeTypeError('symbol_to_string');
+}
+
+function SymbolValueOf() {
+  // NOTE: Both Symbol objects and values can enter here as
+  // 'this'. This is not as dictated by ECMA-262.
+  if (!IS_SYMBOL(this) && !IS_SYMBOL_WRAPPER(this)) {
+    throw MakeTypeError(
+        'incompatible_method_receiver', ["Symbol.prototype.valueOf", this]);
+  }
+  return %_ValueOf(this);
+}
+
+//-------------------------------------------------------------------
+
+function SetUpSymbol() {
+  %CheckIsBootstrapping();
+
+  %SetCode($Symbol, SymbolConstructor);
+  %FunctionSetPrototype($Symbol, new $Symbol());
+  %SetProperty($Symbol.prototype, "constructor", $Symbol, DONT_ENUM);
+
+  InstallFunctions($Symbol.prototype, DONT_ENUM, $Array(
+    "toString", SymbolToString,
+    "valueOf", SymbolValueOf
+  ));
+}
+
+SetUpSymbol();