isCompleted property Null safety
Whether the future has been completed.
Reflects whether complete or completeError has been called.
A true value doesn't necessarily mean that listeners of this future
have been invoked yet, either because the completer usually waits until
a later microtask to propagate the result, or because complete
was called with a future that hasn't completed yet.
When this value is true, complete and completeError must not be
called again.
Implementation
bool get isCompleted;