Enable offline debugging of widget trees

This would allow apps to dump a snapshot of their widget tree and send it over when debugging memory and/or performance issues.

It would also allow us to build tooling to analyze highly complex scenes with 100s of widgets and generate a report.

没有指定文件、测试或入口点。首先梳理 DevTools 当前如何表示 widget 树以及如何处理调试数据，然后定义快照和传输的边界。完成标准应包括对复杂场景的 widget 树进行离线分析和报告生成，包括内存或性能调查。

由索引模型根据 Issue 内容生成。