Add isPromise V8 extras util

test/cctest/test-extra.js

Index: test/cctest/test-extra.js
diff --git a/test/cctest/test-extra.js b/test/cctest/test-extra.js
index 69a7e63221545de7e85a12ccc13fbe12b93748b0..689738c02b346edddcae5d60a5a45fea6d002985 100644
--- a/test/cctest/test-extra.js
+++ b/test/cctest/test-extra.js
@@ -93,7 +93,9 @@
       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
     };
   };
 })