[infra] flutter dashboard's left panel section header is confusing

### Steps to reproduce

In this page:

https://flutter-dashboard.appspot.com/#/presubmit?repo=flutter&pr=190826&sha=626342e5b14f95b9ce6cf71ad4d047cfdef4f4a7&job=Linux_android_emu_vulkan_stable%20android_engine_vulkan_tests

The left panel shows 2 section headers: FUSION and ENGINE.

`Mac mac_unopt` is under FUSION, but `Mac mac_ios_engine` is under engine, which is very confusing.

### Expected results

I'd expect `Mac mac_unopt` (which contains iOS scenario test) to be under engine as well.

### Actual results

`Mac mac_unopt` is under FUSION (not sure what FUSION means, but I assume it's referring to engine + framework in the old 2 repos world?)

### Code sample

NA

### Screenshots or Video

NA

### Logs

NA

### Flutter Doctor output

NA

Start at the linked Flutter dashboard presubmit page and inspect how the left-panel sections classify the Mac jobs. Compare the placement of `Mac mac_unopt` and `Mac mac_ios_engine`; done means the iOS scenario job appears under ENGINE rather than FUSION, with the existing dashboard behavior verified.

Written by the indexing model from the issue text.