Put RegExp js code in strict mode

test/mjsunit/regexp-lastIndex.js

+// Copyright 2016 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.
+
+// lastIndex is set according to funny rules. It is typically set only
+// for global or sticky RegExps, but on a failure to find a match, it is
+// set unconditionally. If a set fails, then it acts as if in strict mode

[Yang, 2016/03/15 06:56:36]

I wonder whether it makes sense to discuss these funny rules in TC39 and fix
them. This is really surprising to me.

+// and throws.
+
+var re = /x/;
+Object.defineProperty(re, 'lastIndex', {writable: false});
+assertThrows(() => re.exec(""), TypeError);
+assertEquals(["x"], re.exec("x"));
+
+var re = /x/y;
+Object.defineProperty(re, 'lastIndex', {writable: false});
+assertThrows(() => re.exec(""), TypeError);
+assertThrows(() => re.exec("x"), TypeError);