util: delay creation of debug context

We need the debug context to be able to inspect promises. However, this is
very expensive and should not be done on default startup.

PR-URL: #2248
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Christopher Monsanto <chris@monsan.to>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/util.js

@@ -2,8 +2,8 @@
 
 const uv = process.binding('uv');
 const Buffer = require('buffer').Buffer;
-const Debug = require('vm').runInDebugContext('Debug');
 const internalUtil = require('internal/util');
+var Debug;
 
 const formatRegExp = /%[sdj%]/g;
 exports.format = function(f) {
@@ -168,6 +168,7 @@ function arrayToHash(array) {
 
 
 function inspectPromise(p) {
+  Debug = Debug || require('vm').runInDebugContext('Debug');
   var mirror = Debug.MakeMirror(p, true);
   if (!mirror.isPromise())
     return null;