Implement the `RegExp.prototype.flags` getter

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);
+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);