Harden %SetIsObserved with RUNTIME_ASSERTs

test/mjsunit/es7/object-observe-runtime.js

+// Copyright 2014 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.
+
+// Flags: --allow-natives-syntax
+
+// These tests are meant to ensure that that the Object.observe runtime
+// functions are hardened.
+
+var obj = {};
+%SetIsObserved(obj);
+assertThrows(function() {
+  %SetIsObserved(obj);
+});
+
+assertThrows(function() {
+  %SetIsObserved(this);
+});