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."

该 issue 没有指出文件、测试或入口点。请明确当前 widget 状态应如何暴露动画控制器和滚动控制器，以及必须显示哪些 RenderObject 细节（例如大小和 constraints）。完成标准应包括已达成一致的范围，以及验证布局调试信息的方法。

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