Make `RegExp.prototype.flags` getter configurable

test/mjsunit/harmony/regexp-flags.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-regexps
 
-delete RegExp.prototype.flags;
 RegExp.prototype.flags = 'setter should be undefined';
 
 assertEquals('', RegExp('').flags);
 assertEquals('', /./.flags);
 assertEquals('gimy', RegExp('', 'ygmi').flags);
 assertEquals('gimy', /foo/ymig.flags);
 
 // TODO(dslomov): When support for the `u` flag is added, uncomment the first
 // line below and remove the second line.
 //assertEquals(RegExp('', 'yumig').flags, 'gimuy');
 assertThrows(function() { RegExp('', 'yumig').flags; }, SyntaxError);
 
 var descriptor = Object.getOwnPropertyDescriptor(RegExp.prototype, 'flags');
-assertFalse(descriptor.configurable);
+assertTrue(descriptor.configurable);
 assertFalse(descriptor.enumerable);
 assertInstanceof(descriptor.get, Function);
 assertEquals(undefined, descriptor.set);
 
 function testGenericFlags(object) {
   return descriptor.get.call(object);
 }
 
 assertEquals('', testGenericFlags({}));
 assertEquals('i', testGenericFlags({ ignoreCase: true }));
 assertEquals('uy', testGenericFlags({ global: 0, sticky: 1, unicode: 1 }));
 assertEquals('m', testGenericFlags({ __proto__: { multiline: true } }));
 assertThrows(function() { testGenericFlags(); }, TypeError);
 assertThrows(function() { testGenericFlags(undefined); }, TypeError);
 assertThrows(function() { testGenericFlags(null); }, TypeError);
 assertThrows(function() { testGenericFlags(true); }, TypeError);
 assertThrows(function() { testGenericFlags(false); }, TypeError);
 assertThrows(function() { testGenericFlags(''); }, TypeError);
 assertThrows(function() { testGenericFlags(42); }, TypeError);
+
+var counter = 0;
+var map = {};
+var object = {
+  get global() {
+    map.g = counter++;
+  },
+  get ignoreCase() {
+    map.i = counter++;
+  },
+  get multiline() {
+    map.m = counter++;
+  },
+  get unicode() {
+    map.u = counter++;
+  },
+  get sticky() {
+    map.y = counter++;
+  }
+};
+testGenericFlags(object);
+assertEquals({ g: 0, i: 1, m: 2, u: 3, y: 4 }, map);