title: Empty value on Polymer computed attribute
body: <p>I am trying to use computed attribute in Polymer but I always get an empty value. In this case I have an attribute in my custom element called datagraph. And I make a post request to a server, get a JSON file, calculate the result and then show it. This is my custom element:</p>
  
  <p>
  
  </p>
  
  <pre><code>&lt;dom-module id="cost-card"&gt;
    &lt;template&gt;
      &lt;style&gt;
        p.datos3{
          color: #10cebc;
          text-align: center;
          font-size: 22px;
          margin-top: 0px;
        }
      &lt;/style&gt;  
      &lt;p class="datos3"&gt;{{datagraph}}&lt;/p&gt;  
    &lt;/template&gt;
    &lt;script&gt;
      Polymer({
        is: "cost-card",
  
        properties:{
      usr:{
        type: Number,
        value: 2
      },
      command:{
        type: String,
        value: "SP_GetCostoCampania"
      },  
      datagraph:{
        type: Number,
        computed: 'getCost(command,usr)'
      }
    },
  
        getCost: function(command,usr){
  
          var options = {
            hostname: 'localhost',
            path: '/',
            port: 8081,
            secure: false,
            method: 'POST',
            headers: {
              'x-powered-by': 'HTTPClient.js'
            },
            'Content-Type': 'application/json'
          }
  
          var innerCost;
  
          var example1 = new HTTPClient(options);
          example1.post("/executeGraph1?command="+ command + "&amp;param1=" + usr, function (err, res, body) {
  
            body = JSON.parse(body);
  
            innerCost = body[0].price * body[0].exchengeRate;
          });
  
          return innerCost;
        }
      });
    &lt;/script&gt;
  &lt;/dom-module&gt;
  </code></pre>
  
  <p>I have an express server running, information is being delivered correctly but the {{datagraph}} tag keeps empty. I think it may be because the post request is an anychronous task, and the value is being delivered later but I have also tried using Promise with the same result.</p>
  
  <p>Does anyone know the proper way to do this?</p>