[regexp] Set static property attributes as in spec proposal

test/mjsunit/regress/regress-5566.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.
+
+// https://github.com/tc39/proposal-regexp-legacy-features#additional-properties-of-the-regexp-constructor
+
+const props = [ "input", "$_"
+              , "lastMatch", "$&"
+              , "lastParen", "$+"
+              , "leftContext", "$`"
+              , "rightContext", "$'"
+              , "$1", "$2", "$3", "$4", "$5", "$6", "$7", "$8", "$9"
+              ];
+
+for (let i = 0; i < props.length; i++) {
+  const prop = props[i];
+  const desc = Object.getOwnPropertyDescriptor(RegExp, prop);
+  assertTrue(desc.configurable, prop);
+  assertFalse(desc.enumerable, prop);
+  assertTrue(desc.get !== undefined, prop);
+
+  // TODO(jgruber): Although the spec proposal specifies setting setters to
+  // undefined, we are not sure that this change would be web-compatible, and
+  // we are intentionally sticking with the old behavior for now.
+  assertTrue(desc.set !== undefined, prop);
+}