Add isPromise V8 extras util

test/cctest/test-extra.js

 // Copyright 2015 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.
 
 (function (global, binding, v8) {
   'use strict';
   binding.testExtraShouldReturnFive = function() {
     return 5;
   };
 
@@ skipping 75 matching lines @@
     let promiseStates = promiseStateToString(new Promise(() => {})) + ' ' +
                         promiseStateToString(fulfilledPromise) + ' ' +
                         promiseStateToString(rejectedPromise);
 
     return {
       privateSymbol: v8.createPrivateSymbol('sym'),
       fulfilledPromise, // should be fulfilled with 1
       fulfilledPromise2, // should be fulfilled with 2
       rejectedPromise, // should be rejected with 3
       rejectedButHandledPromise, // should be rejected but have a handler
-      promiseStates // should be the string "pending fulfilled rejected"
+      promiseStates, // should be the string "pending fulfilled rejected"
+      promiseIsPromise: v8.isPromise(fulfilledPromise), // should be true
+      thenableIsPromise: v8.isPromise({ then() { } })  // should be false
     };
   };
 })