Defer CONST_LEGACY redeclaration errors until runtime in harmony mode

test/mjsunit/regress/regress-4576.js

Index: test/mjsunit/regress/regress-4576.js
diff --git a/test/message/for-in-loop-initializers-destructuring.js b/test/mjsunit/regress/regress-4576.js
similarity index 56%
copy from test/message/for-in-loop-initializers-destructuring.js
copy to test/mjsunit/regress/regress-4576.js
index eab8b81cf371b13c0bf44a48cb106acb1a59badf..c55c69580a5843bb85c84cb76f7f3a197d58ca12 100644
--- a/test/message/for-in-loop-initializers-destructuring.js
+++ b/test/mjsunit/regress/regress-4576.js
@@ -2,8 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
-// Flags: --harmony-destructuring-bind
+// Flags: --harmony-sloppy --legacy-const
 
+// Should trigger a runtime error, not an early error.
 function f() {
-  for (var [x, y] = {} in {});
+  const x;
+  var x;
 }
+assertThrows(f, SyntaxError);