Implement the `RegExp.prototype.flags` getter

src/regexp.js

 // Copyright 2012 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.
 
 // This file relies on the fact that the following declaration has been made
 // in runtime.js:
 // var $Object = global.Object;
 // var $Array = global.Array;
 
 var $RegExp = global.RegExp;
@@ skipping 413 matching lines @@
                                    DONT_DELETE);
   %DefineAccessorPropertyUnchecked($RegExp, "$'", RegExpGetRightContext,
                                    NoOpSetter, DONT_ENUM | DONT_DELETE);
 
   for (var i = 1; i < 10; ++i) {
     %DefineAccessorPropertyUnchecked($RegExp, '$' + i,
                                      RegExpMakeCaptureGetter(i), NoOpSetter,
                                      DONT_DELETE);
   }
   %ToFastProperties($RegExp);
+
+
+  // ES6 draft 12-06-13, section 21.2.5.3
+  // + https://bugs.ecmascript.org/show_bug.cgi?id=3423
+  var RegExpGetFlags = function() {
+    if (!IS_SPEC_OBJECT(this)) {
+      throw MakeTypeError('home_object_non_object',
+                          'RegExp.prototype.flags',
+                          [%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;
+  };
+  //if (harmony_regexps) {

[Dmitry Lomov (no reviews), 2014/12/09 23:02:00]

harmony_regexps should not be used in bootstrapping code (code that executes on
loading natives).
[other uses in this file are ok since they are inside function bodies]

What we typically do is create a separate file, harmony-regexp.js, that is
loaded conditionally under FLAG_harmony_regexp.

+  %DefineAccessorPropertyUnchecked($RegExp.prototype, 'flags', RegExpGetFlags,
+                                   NoOpSetter, DONT_DELETE);
+  %SetNativeFlag(RegExpGetFlags);
+  //}

[mathias, 2014/12/09 22:50:48]

Uncommenting the `if` statement breaks the build. Am I doing something crazy
here?

 }
 
 SetUpRegExp();