[core] Polymer 3 support

[thepassle]

I think this only requires a small change to the static properties getter, since it supports a default value, e.g.:

  static get properties() {
    return {
      mode: {
        type: String,
        value: 'auto'
      },

      data: {
        type: Object,
        notify: true,
        value: function() { return {}; }
      }
    }
  }

Note that objects and arrays are a function that return an object or array