Further ES2015 RegExp spec compliance fixes

test/mjsunit/es6/regexp-tostring.js

Index: test/mjsunit/es6/regexp-tostring.js
diff --git a/test/mjsunit/es6/regexp-tostring.js b/test/mjsunit/es6/regexp-tostring.js
index 3deeeb7ed8894d440d3fabd1172da5203d829c39..23e137c7aa5e0930276f58de43f1521b9f68d9b0 100644
--- a/test/mjsunit/es6/regexp-tostring.js
+++ b/test/mjsunit/es6/regexp-tostring.js
@@ -44,3 +44,14 @@ testThrows(1);
 
 assertEquals("/pattern/flags", RegExp.prototype.toString.call(fake));
 assertEquals(["p", "ps", "f", "fs"], log);
+
+// Monkey-patching is also possible on RegExp instances
+
+let weird = /foo/;
+Object.defineProperty(weird, 'flags', {value: 'bar'});
+Object.defineProperty(weird, 'source', {value: 'baz'});
+assertEquals('/baz/bar', weird.toString());
+
+assertEquals('/(?:)/', RegExp.prototype.toString());
+assertEquals('(?:)', RegExp.prototype.source);
+assertEquals('', RegExp.prototype.flags);