_debugCanPerformMutations assert improvements #87423
Description
Right now we assert(_debugCanPerformMutations) in various places, notably markNeedsLayout. We should instead make _debugCanPerformMutations into some sort of validator function that throws useful error messages for each condition that would otherwise have made the assert fail. For example, I just got this:
══╡ EXCEPTION CAUGHT BY FOUNDATION LIBRARY ╞════════════════════════════════════════════════════════
The following assertion was thrown while dispatching notifications for ListViewSelectionController:
'package:flutter/src/rendering/object.dart': Failed assertion: line 1609 pos 12:
'_debugCanPerformMutations': is not true.
Either the assertion indicates an error in the framework itself, or we should provide substantially
more information in this error message to help you determine and fix the underlying cause.
In either case, please report this assertion by filing a bug on GitHub:
https://github.com/flutter/flutter/issues/new?template=2_bug.md
When the exception was thrown, this was the stack:
#2 RenderObject.markNeedsLayout (package:flutter/src/rendering/object.dart:1609:12)
#3 RenderBox.markNeedsLayout (package:flutter/src/rendering/box.dart:2320:11)
...
...because my RenderObject was disposed at the time I called markNeedsLayout, but to find that out I had to instrument _debugCanPerformMutations to print out which condition was making it return false.