[es6] correct constant_elements backing-store when spread is used

test/mjsunit/harmony/spread-array.js

 // Copyright 2015 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.
 
 // Flags: --harmony-spread-arrays --allow-natives-syntax
 
 (function TestBasics() {
   var a = [1, 2];
   var b = [...a];
   assertArrayEquals([1, 2], b)
 
   assertArrayEquals(['a', 'b', 'c', 'd', 'e', 'f'],
                     ['a', ...'bc', 'd', ...'ef'])
+  assertEquals([1, 2, 3], [1, 2, ...[3]]);
+  assertEquals([1, 2, , 3, 4, 5], [1, 2, ,...[3], 4, 5]);
+  assertEquals(['a', 'b', 'c'], ['a', 'b', ...['c']]);
+  assertEquals(['a', 'b', ,'c', 'd', 'e'], ['a', 'b', , ...['c'],'d', 'e']);
+  assertEquals([{a: true},{b: false},{c: true},{d: false}],
+               [{a: true},{b: false},...[{c: true}],{d: false}]);
 })();
 
 
 var log = [];
 
 function* gen(n) {
   log.push(n, 1);
   yield 1;
   log.push(n, 2);
   yield 2;
@@ skipping 146 matching lines @@
         }
       };
     }
   };
   function f() {
     return [0, ...iter];
   }
 
   assertArrayEquals([0, 1, 2], f());
 });