7175911: Simplify error reporting API in Check.CheckContext interface
Summary: Make error messages generated during Check.checkType more uniform and more scalable
Reviewed-by: jjg, dlsmith

7151492: Encapsulate check logic into Attr.ResultInfo
Summary: ResultInfo class should be used to make attribution code transparent w.r.t. check logic being used
Reviewed-by: jjg, dlsmith

6979683: inconsistent interaction of reference cast with box/unbox conversions leaves out a useful case
Summary: Allow casts which narrow and then unbox.
Reviewed-by: mcimadamore