getFavicon(tab)

gistfile1.js

const tabToPromise = tab => {
  let _then = favUrl => Promise.resolve(Object.defineProperty(tab,'favicon',{value:favUrl}));
  return tab.favicon.then(_then,_then);
}

tabToPromise(tabData).then(bookmark =>
    Request({
      url: DBURL,
      content: JSON.stringify(bookmark),
      contentType: "application/json",
      onComplete: response => {       
        if(response.status==201)
          myButton.state(tab,{icon:ICONRED});
      }
    }).post()
  );