util: optimize promise introspection

Use V8's builtin ObjectIsPromise() to check that the value is a promise
before creating the promise mirror.  Reduces garbage collector strain
in the (common) non-promise case, which is beneficial when inspecting
deep object graphs.

lib/util.js

@@ -3,7 +3,9 @@
 const uv = process.binding('uv');
 const Buffer = require('buffer').Buffer;
 const internalUtil = require('internal/util');
+
 var Debug;
+var ObjectIsPromise;
 
 const formatRegExp = /%[sdj%]/g;
 exports.format = function(f) {
@@ -184,10 +186,15 @@ function getConstructorOf(obj) {
 
 
 function inspectPromise(p) {
-  Debug = Debug || require('vm').runInDebugContext('Debug');
-  var mirror = Debug.MakeMirror(p, true);
-  if (!mirror.isPromise())
+  if (!Debug) {
+    const runInDebugContext = require('vm').runInDebugContext;
+    const result = runInDebugContext('[Debug, ObjectIsPromise]');
+    Debug = result[0];
+    ObjectIsPromise = result[1];
+  }
+  if (!ObjectIsPromise(p))
     return null;
+  const mirror = Debug.MakeMirror(p, true);
   return {status: mirror.status(), value: mirror.promiseValue().value_};
 }