New Twiddle

controllers.application.js

import Ember from 'ember';

const { Controller } = Ember;

export default Ember.Controller.extend({
  promptString: 'regular prompt string',
  promptSafeString: Ember.String.htmlSafe('safe-string prompt string'),
});

templates.application.hbs

<h1>Raw string:</h1>
{{one-way-select
  prompt=promptString
}}

<h1>SafeString:</h1>
{{one-way-select
  prompt=promptSafeString
}}

<h1>Workaround:</h1>
{{one-way-select
  prompt="doesn't matter"
  promptText=promptSafeString
}}

twiddle.json

{
  "version": "0.10.6",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.9.0",
    "ember-data": "2.9.0",
    "ember-template-compiler": "2.9.0",
    "ember-testing": "2.9.0"
  },
  "addons": {
    "ember-one-way-controls": "1.1.1"
  }
}