Add ES2015 RegExp full subclassing semantics behind a flag

test/mjsunit/regexp-string-methods.js

Index: test/mjsunit/regexp-string-methods.js
diff --git a/test/mjsunit/regexp-string-methods.js b/test/mjsunit/regexp-string-methods.js
index 56604a6539e761d592e01f73ae8804649556815d..fa01a33ce4d5e6ef4a850af71f8b4e9a50c7db66 100644
--- a/test/mjsunit/regexp-string-methods.js
+++ b/test/mjsunit/regexp-string-methods.js
@@ -25,6 +25,8 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+// Flags: --no-harmony-regexp-exec
+
 // Regexp shouldn't use String.prototype.slice()
 var s = new String("foo");
 assertEquals("f", s.slice(0,1));