Use %DebugGetProperty in debug mirror to check for Promise.

src/promise.js

Index: src/promise.js
diff --git a/src/promise.js b/src/promise.js
index f4c60d66cacc9538b8aa7d7133481251b3833b0e..959be51a84faaccedb013d3c68855aa274573351 100644
--- a/src/promise.js
+++ b/src/promise.js
@@ -301,12 +301,3 @@ function SetUpPromise() {
 }
 
 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);
-}