<step-button>, a button that can vend promises that resolve when it's clicked

step-button.html

<!-- Call wait() on this button to return a Promise that resolves when the button is clicked. -->
<polymer-element name='step-button'>
  <template><button on-click="{{step}}" disabled?="{{!_resolve}}">Step</button></template>
  <script>
    (function() {
      Polymer('step-button', {
        wait: function() {
          var self = this;
          return new Promise(function(resolve, reject) {
            if (self._resolve) {
              reject('Schedule one step at a time.');
            } else {
              self._resolve = resolve;
            }
          });
        },
        step: function() {
          this._resolve();
          this._resolve = null;
        },
      });
    })();
  </script>
</polymer-element>