Only a single isolate shown in isolate picker when multiple are running

Paraphrased from Discord discussion about memory panel (@srawlins report):

> "There are two isolates running, but I only see one in the isolate picker. Observatory shows several isolate groups, and I see the system isolate group, which the primary DAS code seems to run in. And then my "plugin" isolate group, which is what I'm looking for. (The primary isolate in that isolate group appears to be also called "main.""

从 memory panel 的 isolate picker 开始，使用多个正在运行的 isolate 重现该报告。将 picker 与 Observatory 的 isolate groups 进行比较，包括 system 和 plugin 组；完成标准是每个相关的运行中 isolate 或组都可以被选中。

由索引模型根据 Issue 内容生成。