Finds missing import statements for custom elements

findMissingImports.js

findMissingImports = (element) => {
   if (element.tagName.indexOf('-') != -1 && !customElements.get(element.tagName.toLowerCase())) {
     console.error("Element "+element.tagName+" is not upgraded");
   }
   Array.from(element.children).forEach(e => findMissingImports(e));
   if (element.shadowRoot) {
      Array.from(element.shadowRoot.children).forEach(e => findMissingImports(e));
   }
}