TreeSliver & TreeView support for lazy tree nodes #148358
Description
From flutter/packages#6592 (comment)
Follow up to #147171 and flutter/packages#6592
In the 1D and 2D tree PRs, the entire tree is expected to be provided by the user. Some folks may like to provide nodes lazily, just like the trees allow users to provide widgets for the trees lazily.
This issue is tracking that additional feature request.