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.""