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

test/mjsunit/migrations.js

Index: test/mjsunit/migrations.js
diff --git a/test/mjsunit/migrations.js b/test/mjsunit/migrations.js
index 288bc610312edc2c6ae7eeaf4cabbeb5d794463e..a18d884059baf40cc59294bbef5ed921b0bc20f0 100644
--- a/test/mjsunit/migrations.js
+++ b/test/mjsunit/migrations.js
@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-ayle license that can be
 // found in the LICENSE file.
 
+// Flags: --harmony-object-observe
 // Flags: --allow-natives-syntax --track-fields --expose-gc
 
 var global = Function('return this')();