code: function() {
        // apply controller styles, but only if they are not yet applied
        var scope = this.findStyleScope();
        if (scope && !this.scopeHasNamedStyle(scope, this.localName)) {
          // allow inherited controller styles
          var proto = getPrototypeOf(this), cssText = '';
          while (proto && proto.element) {
            cssText += proto.element.cssTextForScope(STYLE_CONTROLLER_SCOPE);
            proto = getPrototypeOf(proto);
          }
          if (cssText) {
            this.installScopeCssText(cssText, scope);
          }
        }
      }
docs: Installs external stylesheets and <style> elements with the attribute
  polymer-scope='controller' into the scope of element. This is intended
  to be a called during custom element construction.