Migrate debugger panel and eval to use DAP instead of the VM Service

We would like to reduce the work required to support new language features by migrating part of Dart DevTools over to using [DAP.](https://microsoft.github.io/debug-adapter-protocol/)

DAP is the abstract protocol designed by Microsoft to be used between a debugging tool (e.g., VS Code, Dart DevTools) and a debugger (e.g., VM Service).

With Dart DevTools using the DAP interface to represent variables, we will only need to implement features in the Dart DAP package, instead of in both the Dart DAP package and in Dart DevTools.


首先阅读 issue 中链接的 DAP 规范，然后定位 Dart DevTools 中的调试器面板和 eval 实现。确定当前的 VM Service 集成以及所需的 DAP 接口；当这些部分使用 DAP 而不是 VM Service 时，工作就完成了。

由索引模型根据 Issue 内容生成。