Returns an instance of a BooleanCancelable that's already canceled.
      
    
      Builder for BooleanCancelable.
Builder for BooleanCancelable.
is a function that will execute exactly once on canceling.
      
    
      Builder for BooleanCancelable.
      
    
      
      
    
      
      
    
      Returns a BooleanCancelable that can never be canceled.
Returns a BooleanCancelable that can never be canceled.
Useful as a low-overhead instance whose isCanceled value
is always false, thus similar in spirit with alreadyCanceled.