identify path to element's state in application state tree. 

   ### Example:

   #### HTML:

      <template>

        <div>Value A: [[state.valueA]]</div>
        <div>Value B: [[valueB]]</div>

      </template>

   #### JavaScript:

      class MyElement extends UniFlow.StateAware(Polymer.Element) {
        static get is() { return 'my-element'; }

        static get properties() {
          return {
            valueB: String
          }
        }
      }

      customElement.define(MyElement.is, MyEle