observer always fired

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  aFlag: Ember.computed('foo', function() {
    return false;
  }),
  
  aFlagChanged: Ember.observer('aFlag', function () {
    alert('Why do I appear?');
  })
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  foo: 'bar'
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{input value=foo}}
{{my-component foo=foo}}
<br>
<br>

templates.components.my-component.hbs

my component's foo={{foo}}
{{aFlag}}

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}