Layout Debugging - find the current state of a widget

from @jacob314
"Elements from the State object that I often wish we did a better job of displaying to debug layout issues:
State of animation and scroll controllers referenced by the state or widget object.
"state" really on the RenderObject associated with the widget such as size and constraints."

The issue names no files, tests, or entry points. Clarify how the current widget state should expose animation and scroll controllers, and which RenderObject details—such as size and constraints—must be displayed. Done should include an agreed scope and a way to verify the layout-debugging information.

Written by the indexing model from the issue text.