Correctly mark scalarlists as abstract classes with factory constructors.
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

Review URL: https://codereview.chromium.org//11000018

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@12951 260f80e4-7a28-3924-810f-c04153c831b5

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