gistfile1.html

<script>

  Polymer.FooBehavior = {
    properties: {
      bar: {
        type: String,
        readOnly: true
      }
    }
  }

</script>

<script>

  Polymer({
    is: 'x-app',
    behaviors: [Polymer.FooBehavior],
    test: function() {
      console.log(this._setBar); // output is undefined but it shouldn't
    }
  });

</script>