Image Cache changes

[Dan Field]

https://github.com/flutter/flutter/pull/50318 has updated the ImageCache contract to include things like how the image is being tracked in the cache, including whether the cache thinks it is pending, being kept alive, or live somewhere in the tree (but not necessarily in the cache). This involved adding new methods, which broke tests for implementers of ImageCache that did not previously know about these methods.

When I opened the PR, it was a breaking change based on Flutter's compatibility guidelines. During the course of working on it, the tests it broke (which are internal only to Google) went away, and so when it landed it was no longer a breaking change. Because of this, I have not written a migration guide - although there is a design doc on the PR if you want more details.

If you are concerned about changes to the ImageCache like this, please consider contributing some tests to https://github.com/flutter/tests so we will know to treat them as breaking changes.  The same is true for other areas of Flutter's API surface that you care about.

Thanks,

Dan