Format on Dart 2.10.4, only check formatting on stable runs

[greglittlefield-wf]

Motivation

CI was failing on every commit because the code was formatted under Dart 2.9, and CI was only skipping formatting on dev runs and not the Dart 2.10 stable run.

Previously, stable runs were broken due to build issues, but now that that's fixed, we can format on stable.

Solution

• Format on Dart 2.10.4 (stable)
• Only check formatting on stable runs

QA steps

• Verify all Travis builds are green