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 flutter/flutter#160616

[dart-github-bot]

Summary: User wants dart fix support for dart:ui's Color class deprecations. They tried adding fix_data, but it didn't work; seeking guidance on correct setup.

[bwilkerson]

The tool currently looks for a fix_data.yaml only in the root of the package defining the library containing the class in question. For dart: libraries there is no package, so we had to special case a location for dart: libraries in the SDK. I don't believe that we thought about having a second location for dart: libraries from the engine.

So the right answer here is probably to have a second place to look for fixes related to dart: libraries.

[Piinks]

So the right answer here is probably to have a second place to look for fixes related to dart: libraries.

That would be great @bwilkerson! If so, we'd probably not put the fix data in the engine like in the linked PR. Instead, putting it with all of the other fix data we already have in package:flutter.