In the Dart VM, isolates are always spawned into groups - either a new group as the "main" isolate or into an existing group as a "child" isolate.

In the engine tree, we refer to the UI isolate frequently as the "root" isolate. This probably made sense before isolate groups, and maybe continued to make sense before we allowed for "lightweight engines" to spawn UI isolates in the same groups together - but it now is confusing.

We should clean up the terminology so that we don't refer to these as root isolates anymore, but instead as UI isolates. We should also consider making it clearer in the code/naming conventions as to whether a UI isolate is spawned as a main isolate for a group or a child isolate in the group.

/cc @aam if there's anything else you think we should add to this.