Summary:
I have a use case where I would like to check if the
AsyncPipe is still active without pumping something in to
write(). Could set up an onClosed callback, but this would introduce
more complexity.

This adds a function to determine if the queue inside the AsyncPipe
still exists.

Reviewed By: iahs

Differential Revision: D28015007

fbshipit-source-id: 1ba37466218abd2f42eab1d8ea5677c3945e31f5