code: function register(val) {
        var content = void 0;
        var template = document.createElement('template');
        template.innerHTML = val;
        if (template.content) {
          content = template.content; // eslint-disable-line prefer-destructuring
        } else {
          content = document.createDocumentFragment();
          while (template.firstChild) {
            content.appendChild(template.firstChild);
          }
        }
        document.importNode(content, true);
      }
docs: Create a `<template>` element to hold `<dom-module>` content.
  This bit of code will execute in the context of the main document,
  calling `importNode` on the `<template>`, which in turn triggers
  the lifecycle of the `<dom-module>` and allows it to insert its
  content into Polymer's global module map. When a Polymer element
  boots up it will fetch its template from this module map.
  https://github.com/Polymer/polymer/blob/master/lib/mixins/element-mixin.html#L501-L538
  @param {string} val A `<dom-module>` as an HTML string