TimelineTask constructor
Create a task. The task ID will be set by the system.
If parent is provided, the parent's task ID is provided as argument
'parentId' when start is called. In DevTools, this argument will result
in this TimelineTask being linked to the parent TimelineTask.
Implementation
TimelineTask({TimelineTask parent})
: _parent = parent,
_taskId = _getNextAsyncId() {}