Implement ES6 @@isConcatSpreadable

src/runtime.js

 // Copyright 2006-2008 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 files contains runtime support implemented in JavaScript.
 
 // CAUTION: Some of the functions specified in this file are called
 // directly from compiled code. These are the functions with names in
 // ALL CAPS. The compiled code passes the first argument in 'this' and
 // it does not push the function onto the stack. This means that you
@@ skipping 596 matching lines @@
 // Returns if the given x is a primitive value - not an object or a
 // function.
 function IsPrimitive(x) {
   // Even though the type of null is "object", null is still
   // considered a primitive value. IS_SPEC_OBJECT handles this correctly
   // (i.e., it will return false if x is null).
   return !IS_SPEC_OBJECT(x);
 }
 
 
+// ES6, draft 10-14-14, section 22.1.3.1.1
+function IsConcatSpreadable(O) {
+  if (!IS_SPEC_OBJECT(O)) return false;
+  var spreadable = O[symbolIsConcatSpreadable];
+  if (IS_UNDEFINED(spreadable)) return IS_ARRAY(O);
+  return ToBoolean(spreadable);
+}
+
+
 // ECMA-262, section 8.6.2.6, page 28.
 function DefaultNumber(x) {
   if (!IS_SYMBOL_WRAPPER(x)) {
     var valueOf = x.valueOf;
     if (IS_SPEC_FUNCTION(valueOf)) {
       var v = %_CallFunction(x, valueOf);
       if (%IsPrimitive(v)) return v;
     }
 
     var toString = x.toString;
@@ skipping 29 matching lines @@
   return i;
 }
 
 
 // NOTE: Setting the prototype for Array must take place as early as
 // possible due to code generation for array literals.  When
 // generating code for a array literal a boilerplate array is created
 // that is cloned when running the code.  It is essential that the
 // boilerplate gets the right prototype.
 %FunctionSetPrototype($Array, new $Array(0));