Review ApiCompatbility diff

[KathleenDollard]

The ApiCompatibility tests describe the API shape and ensure there are no accidental changes.

We took these tests off-line in Powderhouse as we were making the initial changes. Reviewing now will let us clarify the shape of the core API and identify changes that are still needed.

This is likely to result in a document in the proposals folder that clarifies the changes and the desired API.