ES6: String(symbol) should work like symbol.toString

src/string.js

 // Copyright 2012 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.
 
 // This file relies on the fact that the following declaration has been made
 // in runtime.js:
 // var $String = global.String;
 
 // -------------------------------------------------------------------
 
 function StringConstructor(x) {
-  var value = %_ArgumentsLength() == 0 ? '' : TO_STRING_INLINE(x);
+  if (%_ArgumentsLength() == 0) x = '';
   if (%_IsConstructCall()) {
-    %_SetValueOf(this, value);
+    %_SetValueOf(this, TO_STRING_INLINE(x));
   } else {
-    return value;
+    return IS_SYMBOL(x) ?
+        %_CallFunction(x, SymbolToString) : TO_STRING_INLINE(x);
   }
 }
 
 
 // ECMA-262 section 15.5.4.2
 function StringToString() {
   if (!IS_STRING(this) && !IS_STRING_WRAPPER(this)) {
     throw new $TypeError('String.prototype.toString is not generic');
   }
   return %_ValueOf(this);
@@ skipping 948 matching lines @@
     "fixed", StringFixed,
     "italics", StringItalics,
     "small", StringSmall,
     "strike", StringStrike,
     "sub", StringSub,
     "sup", StringSup
   ));
 }
 
 SetUpString();