It's simple.
Read example.
let lock = ; let { let awaitEnd = ; ; console; let endPromise = await awaitEndpromise; console;}; ;
Start! 1506544867
End! 1506544870
It's work.
let { let awaitEnd = ; awaitEnd; console; let endPromise = await awaitEndpromise; console;}; ;
Start! 1506545013174
End! 1506545013174