CancelableOperation<T>.fromValue  constructor 
      
      CancelableOperation<T>.fromValue (
    
- T value
Creates a CancelableOperation which completes to value.
Canceling this operation does nothing.
Calling this constructor is equivalent to creating a
CancelableCompleter and completing it with value.
Implementation
factory CancelableOperation.fromValue(T value) =>
    (CancelableCompleter<T>()..complete(value)).operation;