Generator functions can't access properties

I have a generator function that's nested in a QML element like this

Rectangle {
   id: fileContainer
   property real a: 0

   function refreshA() {
      AsyncJS.coro(function*() { a = yield getResultsAsync(); });
   }
}

AsyncJS.coro executes the generator whenever necessary when Promises resolve. But it turns out that both 'fileContainer' and 'a' is not visible within the generator. When I pass 'fileContainer' as a parameter into the generator it works, but is very cumbersome and doesn't support unqualified lookup of property and id names.