harmony-scoping: make assignment to 'const' a late error.

test/mjsunit/regress/regress-2506.js

 // Copyright 2014 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-scoping
 
 'use strict';
 
 // Top-level code
 let s = 0;
@@ skipping 28 matching lines @@
 }
 assertEquals(2, s);
 
 
 s = "";
 for (const x in [1,2,3]) {
   s += x;
 }
 assertEquals("012", s);
 
-assertThrows("'use strict'; for (const x in [1,2,3]) { x++ }", SyntaxError);
+assertThrows("'use strict'; for (const x in [1,2,3]) { x++ }", TypeError);
 
 // Function scope
 (function() {
   let s = 0;
   for (const x of [1,2,3]) {
     s += x;
   }
   assertEquals(6, s);
 
   let x = 1;
   s = 0;
   for (const x of [x, x+1, x+2]) {
     s += x;
   }
   assertEquals(6, s);
 
   s = 0;
   var q = 1;
   for (const q of [q, q+1, q+2]) {
     s += q;
   }
   assertEquals(6, s);
 
   s = "";
   for (const x in [1,2,3]) {
     s += x;
   }
   assertEquals("012", s);
 }());