Dart fix support for `dart:X` libraries

[Piinks]

I noticed some deprecations in the Color class were causing some pain for users, so I tried to see if we could add dart fix support for dart:ui.

First I tried setting it up like we have in the framework, with a fix_data directory in the lib/ folder at: engine/src/flutter/lib/ Then I tried moving the fix_data to engine/src/flutter/lib/ui/, but that did not work either. Am I holding it wrong? Can dart fix work in these libraries?

See small fix written in https://github.com/flutter/flutter/pull/160616