Eliminate unnecessary factory constructors in abstract core classes

Eliminate unnecessary factory constructors in abstract core classes

It is now a runtime error to instantiate an explicitly marked
abstract class, so these factory constructors are no longer
necessary.
Committed: https://code.google.com/p/dart/source/detail?r=11965

[hausner, 2012-09-06 16:12:23]

Aargh, forgot to send this out yesterday.

[regis, 2012-09-06 16:14:24]

LGTM

[hausner, 2012-09-06 16:34:47]

Thank you.