Add a few readonly declarations.

All consumers of these APIs compile with the current set of readonly properties. It's best to make the interfaces as strict as possible right now, for a few reasons:

 1) When I get objects of a particular interface, I don't expect its properties to change. Mutability is harder to reason about than immutability.
 2) Open-closed principle: it's easier to move from a restrictive declaration to a less restrictive declaration than the other way around.