gistfile1.html

<dom-module id="x-app">

  <script>

    Polymer({

      is: 'x-app',

      properties: {

        foo: {
          type: String,
          value: 'foo'
        },

        bar: String,

        zot: String

      },

      observers: [
        '_observeBarZot(bar, zot)'
      ],

      _observeBarZot() {
        console.log(this.foo, this.bar, this.zot);  // undefined "bar" "zot"
      }

    });

  </script>

</dom-module>