Add a --harmony-object-observe runtime flag (on by default)

test/mjsunit/regress/regress-356589.js

Index: test/mjsunit/regress/regress-356589.js
diff --git a/test/mjsunit/regress/regress-356589.js b/test/mjsunit/regress/regress-356589.js
index f93c5456407e6f1a433aa2c63d84f0749489a19b..a47f51bac1bf7541ad42ed8af8b2f285ebea0196 100644
--- a/test/mjsunit/regress/regress-356589.js
+++ b/test/mjsunit/regress/regress-356589.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: --harmony-object-observe
+
 // This test passes if it does not crash in debug mode
 
 arr = ['a', 'b', 'c', 'd'];