A cancel token that is passed into cancellable operations. If cancel is called, the promise will be rejected with an instance of support/Cancellable!Cancellation.
Signals cancellation to asynchronous operations that are associated with the cancel token.
Optionalreason: unknownExtra data signifying the reason for cancellation. This will part of the the support/Cancellable!Cancellation that is propagated from the cancel token.
Allows cancellation of asynchronous operations.