Correctly mark scalarlists as abstract classes with factory

Correctly mark scalarlists as abstract classes with factory
constructors.

The marking as abstract avoids warnings about non-implemented
inherited methods for the scalarlists on construction.

Once the classes are abstract the constructors have to be
factory constructors which they are anyway.

R=sgjesse@google.com
BUG=dartbug.com/5469

Committed: https://code.google.com/p/dart/source/detail?r=12951

[Søren Gjesse, 2012-09-27 12:31:16]

lgtm