[strong] Forbid var.

test/mjsunit/strong/var-let-const.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: --strong-mode
+
+(function NoVar() {
+  assertThrows("'use strong'; var x = 0;", SyntaxError);
+  assertThrows("'use strong'; for(var i = 0; i < 10; ++i) { };", SyntaxError);
+})();
+
+
+(function LetIsOkay() {
+  assertTrue(eval("'use strong'; let x = 0; x === 0;"));
+  assertTrue(eval("'use strong'; for(let i = 0; i < 10; ++i) { } 0 === 0;"));
+})();
+
+
+(function ConstIsOkay() {
+  assertTrue(eval("'use strong'; const x = 0; x === 0;"));
+  assertTrue(eval("'use strong'; for(const i = 0; false;) { } 0 === 0;"));
+})();