VM Tools page has several bugs when debugging a Flutter test

Repro: run a flutter test with flutter test test/foo_test.dart --start-paused. Copy the VM service URI and connect to DevTools.

bugs:
- data doesn't load on initial selection of vm tabs. If it is waiting on something, there is no indicator
- switching isolates or clicking refresh throws exceptions
- the page feels generally broken, difficult to trust the state

@bkonyi

使用 `flutter test test/foo_test.dart --start-paused` 重现该问题，复制 VM service URI，并连接到 DevTools。在选择 VM 标签页、切换 isolate 和刷新时检查 VM Tools 页面；当初始数据加载完成或显示等待指示器，并且这些交互不再抛出异常或留下不可信的状态时，即表示完成。

由索引模型根据 Issue 内容生成。