<dom-module id="my-app">
 <template>
   <paper-button on-click="buttonError">Error Test</paper-button>
   <paper-toast id="error" text="you've messed up"></paper-toast>
 </template>
 <script>
  Polymer({
   is: "my-app",
   buttonError: function(event) {
    this.$.error.show(); //something does not feel right here
   }
  });
 </script>
</dom-module>

buttonError: function(event) {
        alert('weird');
        this.$.error.show(); //something does not feel right here
       }