Fetch API cannot load https://api.vk.com/method/wall.get?owner_id=-1&count=1&v=5.64. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'https://www.google.com.ua' is therefore not allowed access. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.
The FetchEvent for "https://api.vk.com/method/wall.get?owner_id=-1&count=1&v=5.64" resulted in a network error response: the promise was rejected.
newtab-serviceworker.js:1 Uncaught (in promise) TypeError: Failed to fetch
VM912:11 GET https://api.vk.com/method/wall.get?owner_id=-1&count=1&v=5.64 net::ERR_FAILED
(anonymous) @ VM912:11
VM912:9 Ошибка 0
Ваш пример хорош, правда не понимаю почему у Вас работает, а у меня нет.