title: Variable inside a class is not defined NodeJS
body: <p>I am develping an application to send emails with nodeJs 6.9 and express 4.
  I am also using a library called powerdrill wich is the library that calls Mandrill APIs.</p>
  
  <p>I am trying to use the message variable which is the varible that use the powerdrill library to create the object and send the email. I am using this variable inside a method but is throwing an error.</p>
  
  <pre><code>'use strict';    
  var config = require('config'),
      queries = require('api/queries'),
      Message = require('powerdrill').Message;
  
  
  class MandrillService{
  
      constructor(config, Message) {
          this.config = config;
          this.Message = Message;
      }
  
      welcome(){
          //console.log(req.body);
          message = new Message();
          message.apiKey(config.mandrillKey)
            .subject('Welcom')
            .template('welcome-email')
            .from(config.mandrilEmail)
            .to('david@gmail.com')
            .tag('complex')
            .globalMergeVar('VERIFY_EMAIL','link')
            .send(function(err, resp) {
            });
      }
  }
  
  module.exports = new MandrillService(config);
  </code></pre>
  
  <p>error:</p>
  
  <pre><code>Unhandled rejection ReferenceError: message is not defined
          at MandrillService.welcome (/home/user/platform/services/mandrillService.js:28:17)
          at /home/user/platform/services/notificationService.js:222:22
          at Promise._execute (/home/user//platform/node_modules/bluebird/js/release/debuggability.js:303:9)
          at Promise._resolveFromExecutor (/home/user/platform/node_modules/bluebird/js/release/promise.js:483:18)
  </code></pre>
  
  <p><strong>I don´t know what is the behavior inside a class but if I try to call the method using express inside a post method, and using it like follows the code works fine.</strong></p>
  
  <pre><code>var express = require('express'),
  router = express.Router(),
  config = require('config'),
  Message = require('powerdrill').Message;
  
  router.post('/welcome',function(req,res,next){
    message = new Message();
    message.apiKey(config.mandrillKey)
      .subject(req.body.subject)
      .template(req.body.template)
      .from(config.mandrilEmail)
      .to(req.body.to)
      .tag('complex')
      .globalMergeVar('VERIFY_EMAIL',req.body.linkVefifyEmail)
      .send(function(err, resp) 
        res.send(resp).end();
      });
  });
  </code></pre>