Use %DebugGetProperty in debug mirror to check for Promise.

src/promise.js

 // Copyright 2012 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.
 
 "use strict";
 
 // This file relies on the fact that the following declaration has been made
 // in runtime.js:
 // var $Object = global.Object
 // var $WeakMap = global.WeakMap
@@ skipping 283 matching lines @@
     "resolve", PromiseCast
   ]);
   InstallFunctions($Promise.prototype, DONT_ENUM, [
     "chain", PromiseChain,
     "then", PromiseThen,
     "catch", PromiseCatch
   ]);
 }
 
 SetUpPromise();
-
-// Functions to expose promise details to the debugger.
-function GetPromiseStatus(promise) {
-  return GET_PRIVATE(promise, promiseStatus);
-}
-
-function GetPromiseValue(promise) {
-  return GET_PRIVATE(promise, promiseValue);
-}