ember.js: 1.12 instanceInitializer deferReadiness does not work

See on SO: http://stackoverflow.com/questions/30276489/ember-1-12-0-instance-initializer-deferreadiness-method-doesnt-exist

The example in http://emberjs.com/blog/2015/05/13/ember-1-12-released.html mentions that deferReadiness is available on the instance, but it is not.

// app/instance-initializers/sleep.js

export function initialize(application) {
  application.deferReadiness();
  // Wait 3s before continuing to boot the app
  Ember.run.later(application, 'advanceReadiness', 3000);
}

export default {
  name: 'sleep',
  initialize: initialize
};

How can we use deferReadiness in 1.12?