call method
Invokes the task's underlying async function with params,
returning the eventual TResult. Behaviour under concurrent
calls depends on the TaskStrategy (see class-level docs).
Throws TaskError synchronously when called after dispose.
Throws a caller-supplied error of type TError through the
returned future when the underlying function fails; non-TError
throws propagate as-is (and revert state to TaskIdle so
observers don't see a stale TaskPending).
Implementation
@override
Future<TResult> call([void params]) => super.call(null);