File flickers in debugger during hot restart

If there are multiple isolates, like when vm mode is enabled, the file flickers when doing a hot restart.

This is because we switch to the other isolate while the main isolate restart. Then when it comes back up, we switch back to the main isolate.

This was originally introduced by fixing https://github.com/flutter/devtools/issues/5017

It didn't make sense to fully address this issue until we have a reliable way to determine if we are in the middle of a hot restart or not though.

As such we filed this issue, asking for events to to sent for the hot restart lifecycle. https://github.com/flutter/flutter/issues/134470

Once that issue has a resolution then we can develop a hot restart lifecycle state, and base a flickerless behaviour off of that.

首先跟进 flutter/flutter#134470，该 issue 跟踪了此 issue 所需的缺失 hot restart 生命周期事件。事件可用后，检查 hot restart 期间 debugger 的 isolate 切换行为，并确认存在多个 isolate 时活动文件不再闪烁。

由索引模型根据 Issue 内容生成。