code: function() {
        if (this.templateInstance && this.templateInstance.model) {
          console.warn('Attributes on ' + this.localName + ' were data bound ' +
              'prior to Polymer upgrading the element. This may result in ' +
              'incorrect binding types.');
        }
        this.created();
        this.prepareElement();
        if (!this.ownerDocument.isStagingDocument) {
          this.makeElementReady();
        }
      }
docs: Low-level lifecycle method called as part of standard Custom Elements
  operation. Polymer implements this method to provide basic default
  functionality. For custom create-time tasks, implement `created`
  instead, which is called immediately after `createdCallback`.
  
  @method createdCallback