message: Add missing require for MslConstants.
diff: var BlockingQueue = require('../util/BlockingQueue.js');
  var MslEncoderFormat = require('../io/MslEncoderFormat.js');
  var TextEncoding = require('../util/TextEncoding.js');
  + var MslConstants = require('../MslConstants.js');
  /**
  * Interface for getting an HTTP response given a request.
  