mixins.ts

import {ɵrenderTemplate as renderTemplate} from '@angular/core'

export function withNgTemplate(Base = HTMLElement){
  return class NgTemplateElement extends HTMLElement {
    
    _renderRoot: HTMLElement | ShadowRoot;
    _host:any;
    _renderer(root:HTMLElement | ShadowRoot, renderFn:any){
      renderFn();
    }
    _render(){
      this._host = renderTemplate(this._renderRoot as HTMLElement, (this.constructor as any).ngTemplate, this, {
        createRenderer(){ return document; }
      }, this._host, null, null, null );
    }

    detectChanges(){
      this._renderer(this._renderRoot, () => this._render && this._render());
    }
  }
}

use.ts

import {withNgTemplate} from '@angular/core'

//@Template({})
class MyElement extends withNgTemplate(){

}

customElements.define('x-foo', MyElement);