Streamline the CPS IR Visitor interface.

Streamline the CPS IR Visitor interface.

Previously, the visitor interface combined two things: (1) the
interface for visiting concrete IR instruction classes and (2) a
default implementation that called the visit function for the
superclass explicitly (but not through super).  For most classes that
extended Visitor this inherited implementation was used to throw an
exception if a visit method for a new class was forgotten.

Instead, it's better to make this class purely an interface which will
give a static warning when a visit method is forgotten.  This change
forces us to implement methods that previously had an inherited (not
necessary correct) implementation.

BUG=
R=asgerf@google.com, karlklose@google.com

Committed: https://code.google.com/p/dart/source/detail?r=44235

[Kevin Millikin (Google), 2015-03-04 11:34:59]

kmillikin@google.com changed reviewers:
+ asgerf@google.com, karlklose@chromium.org

[asgerf, 2015-03-04 11:45:44]

Very nice. LGTM!

[karlklose, 2015-03-04 12:58:16]

karlklose@google.com changed reviewers:
+ karlklose@google.com

[karlklose, 2015-03-04 12:58:17]

LGTM.

Consider having visit-methods that are not intended to be called throw/call
internalError.

[Kevin Millikin (Google), 2015-03-04 13:31:04]

On 2015/03/04 12:58:17, karlklose wrote:
> LGTM.
> 
> Consider having visit-methods that are not intended to be called throw/call
> internalError.

Done.

[Kevin Millikin (Google), 2015-03-04 14:32:17]

Committed patchset #2 (id:20001) manually as 44235 (presubmit successful).