insertAt

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  lastPost: 0,
  
  init() {
    this.data = {
      entities: []
    };
  },
  
  actions: {
    addPost() {
      this.get('data.entities').insertAt(0, this.get('lastPost'));
      this.incrementProperty('lastPost');
    }
  }
});

templates.application.hbs

All posts:
<ul>
{{#each data.entities as |post|}}
	<li>{{post}}</li>
{{/each}}
</ul>

<button {{action 'addPost'}}>Add post</button>

twiddle.json

{
  "version": "0.7.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.4.3/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.4.3/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.4.3/ember-template-compiler.js"
  }
}