lazy-load tls (nodejs#1186)

Author: benjamingr

Diff for: lib/core/connect.js

@@ -1,10 +1,10 @@
 'use strict'
 
 const net = require('net')
-const tls = require('tls')
 const assert = require('assert')
 const util = require('./util')
 const { InvalidArgumentError, ConnectTimeoutError } = require('./errors')
+let tls // include tls conditionally since it is not always available
 
 // TODO: session re-use does not wait for the first
 // connection to resolve the session and might therefore
@@ -24,6 +24,9 @@ function buildConnector ({ maxCachedSessions, socketPath, timeout, ...opts }) {
   return function connect ({ hostname, host, protocol, port, servername }, callback) {
     let socket
     if (protocol === 'https:') {
+      if (!tls) {
+        tls = require('tls') 
+      }
       servername = servername || options.servername || util.getServerName(host) || null
 
       const sessionKey = servername || hostname