Add a "tab" strip for open dart source code windows.

It'd be great if we can have a "tab" strip for open source code tabs, similar to what chrome inspector has today: https://snipboard.io/tNep7n.jpg

Or even if there was a dropdown menu / 3dot menu to show previously opened tabs.
Right now we're restricted to either re-searching for the file via ctrl-p, or clicking the left/right arrows.

首先定位 DevTools 中的源代码窗口导航以及现有的左/右箭头行为。比较 issue 中描述的标签栏和下拉菜单方案，然后将完成定义为：用户无需重新搜索即可重新访问已打开的 Dart 源代码窗口；未指定文件或测试。

由索引模型根据 Issue 内容生成。