Fix error message for Object.observe accept argument

test/mjsunit/es7/object-observe.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1806 matching lines @@
   fun.name = 'c';  // Same, no update.
   Object.deliverChangeRecords(observer.callback);
   observer.assertCallbackRecords([
     { object: fun, type: 'update', name: 'name', oldValue: 'fun' },
     { object: fun, type: 'reconfigure', name: 'name'},
     { object: fun, type: 'update', name: 'name', oldValue: 'a' },
     { object: fun, type: 'delete', name: 'name', oldValue: 'b' },
     { object: fun, type: 'add', name: 'name' },
   ]);
 })();
+
+
+(function TestObserveInvalidAcceptMessage() {
+  var ex;
+  try {
+    Object.observe({}, function(){}, "not an object");
+  } catch (e) {
+    ex = e;
+  }
+  assertInstanceof(ex, TypeError);
+  assertEquals("Third argument to Object.observe must be an array of strings.",
+               ex.message);
+})()