[proxies] Make Array.prototype.concat work correctly with proxies.

src/js/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'.
 
@@ skipping 201 matching lines @@
 function AddIndexedProperty(obj, index, value) {
   if (index === TO_UINT32(index) && index !== kMaxUint32) {
     %AddElement(obj, index, value);
   } else {
     %AddNamedProperty(obj, TO_STRING(index), value, NONE);
   }
 }
 %SetForceInlineFlag(AddIndexedProperty);
 
 
-// ES6, draft 10-14-14, section 22.1.3.1.1
-function IsConcatSpreadable(O) {
-  if (!IS_SPEC_OBJECT(O)) return false;
-  var spreadable = O[isConcatSpreadableSymbol];
-  if (IS_UNDEFINED(spreadable)) return IS_ARRAY(O);
-  return TO_BOOLEAN(spreadable);
-}
-
-
 function ToPositiveInteger(x, rangeErrorIndex) {
   var i = TO_INTEGER_MAP_MINUS_ZERO(x);
   if (i < 0) throw MakeRangeError(rangeErrorIndex);
   return i;
 }
 
 
 function MaxSimple(a, b) {
   return a > b ? a : b;
 }
@@ skipping 33 matching lines @@
   "concat_iterable_to_array_builtin", CONCAT_ITERABLE_TO_ARRAY,
   "reflect_apply_prepare_builtin", REFLECT_APPLY_PREPARE,
   "reflect_construct_prepare_builtin", REFLECT_CONSTRUCT_PREPARE,
 ]);
 
 %InstallToContext([
   "concat_iterable_to_array", ConcatIterableToArray,
 ]);
 
 })