deps: Await/take flutter_color_models release that accommodates wide-gamut adjustments to Flutter's Color

[chrisbobbe]

A Flutter change to the Color API stumbled on some failures in the Zulip tests, which (since #239) are registered in Flutter's "customer tests" suite: flutter/flutter#154123

A Flutter maintainer kindly investigated and found that our dependency flutter_color_models and its dependency color_models needed some changes to accommodate the API change. They made a PR to those projects and asked us to make the Zulip tests pass again by adding dependency overrides pointing to their fork, until the projects incorporate the changes and make a release.

I'm working on a PR with those dependency overrides. This issue is to track removing those overrides and just pointing to versions of those dependencies that accommodate the changed Color API, once such versions are published.

[chrisbobbe]

I'm working on a PR with those dependency overrides.

@PIG208, normally this would go through Greg, like everything else, but he's on vacation and off the grid, and this will need to be merged by Thursday morning; see flutter/flutter#154123. Part of our end of the bargain for having our tests included in Flutter's "customer tests" is that we're responsive when changes need to be made. So I'd appreciate your timely review of my PR once I've made it. 🙂

[PIG208]

Thanks for the heads-up. Will prioritize reviewing this once it is ready.

[chrisbobbe]

OK, sent #920.