[Checkers] AnnotatedTypeMirror is not sufficiently unique

[Michael Ernst]

Mahmood-

> The stored element is not meant to be part of the identity of the type.
> 
> Currently, there methods to go from Tree and Element (and visa versa)  
> and from both to type.  We found it useful to have a method to go from  
> a type to the element generating it, particularly for method and type  
> parameter types.  However, getElement() is intentionally left  
> undocumented as to when the element is available to discourage using  
> it outside the framework until we come up with better plan.

This sounds like a hack.  Do you have a plan for fixing it?  Should we try
to brainstorm one?

                    -Mike