prepareTemplate(template, elementName) {
    this.ensure();
    if (elementHasBuiltCss(template)) {
      return;
    }
    templateMap[elementName] = template;
    let ast = applyShim.transformTemplate(template, elementName);
    // save original style ast to use for revalidating instances
    template['_styleAst'] = ast;
  }
@param {!HTMLTemplateElement} template
@param {string} elementName