A small copy of Polymer's dom-module.

dom-module.html

<script>
  class DomModule extends HTMLElement {
    constructor() {
      super();
    }

    connectedCallback() {
      this.register();
    }

    register() {
      this.constructor.loaded[this.id] = this;
    }

    static import(id, selector) {
      let m = this.loaded[id];
      if(m && selector) {
        return m.querySelector(selector);
      }
      return m;
    }
  }

  DomModule.loaded = {};
  customElements.define('dom-module', DomModule);
</script>