Implement the `RegExp.prototype.flags` getter

src/harmony-regexp.js

Index: src/harmony-regexp.js
diff --git a/src/harmony-regexp.js b/src/harmony-regexp.js
new file mode 100644
index 0000000000000000000000000000000000000000..a2b172983ec34df4e2e234fa6049d4ed858fc824
--- /dev/null
+++ b/src/harmony-regexp.js
@@ -0,0 +1,35 @@
+// 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.
+
+'use strict';
+
+var $RegExp = global.RegExp;
+
+// -------------------------------------------------------------------
+
+// ES6 draft 12-06-13, section 21.2.5.3
+// + https://bugs.ecmascript.org/show_bug.cgi?id=3423
+function RegExpGetFlags() {
+  if (!IS_SPEC_OBJECT(this)) {
+    throw MakeTypeError('flags_getter_non_object',
+                        [%ToString(this)]);
+  }
+  var result = '';
+  if (this.global) result += 'g';
+  if (this.ignoreCase) result += 'i';
+  if (this.multiline) result += 'm';
+  if (this.unicode) result += 'u';
+  if (this.sticky) result += 'y';
+  return result;
+}
+
+function ExtendRegExpPrototype() {
+  %CheckIsBootstrapping();
+
+  %DefineAccessorPropertyUnchecked($RegExp.prototype, 'flags', RegExpGetFlags,
+                                   null, DONT_ENUM | DONT_DELETE);
+  %SetNativeFlag(RegExpGetFlags);
+}
+
+ExtendRegExpPrototype();