Apply stub_ui API conformance check in both directions

[jonahwilliams]

Currently this only applies dart:ui to stub_ui, but we could move the other direction if we're able to move most platform specific logic into _engine, possibly with a whitelist.

[zanderso]

@jonahwilliams Could you provide some more detail about what needs to be done and why?

[jonahwilliams]

Right now we only check that classes and methods that exist in dart:ui also exist in the web implementation, but not that classes and methods that exist in the web implementation exist in dart:ui. This was initially due to the fact that there are several required webOnly* APIs which need to exist, but should probably be cleaned up with an explicit list of these methods to prevent other mistakes.

[jonahwilliams]

I'm not currently working on this

[github-actions]

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