remove serialization. it's moved to github

pkg/serialization/README.md

-A general-purpose serialization facility for Dart Objects.
-
-This provides the ability to save and restore objects to pluggable
-Formats using pluggable Rules.
-These rules can use mirrors or be hard-coded. The main principle
-is using only public APIs on the serialized objects, so changes to the
-internal representation do not break previous serializations. It also handles
-cycles, different representations, filling in known objects on the 
-receiving side, and other issues. It is not as much intended for
-APIs using JSON to pass acyclic structures without class information,
-and is fairly heavweight and expensive for doing that compared to simpler
-approaches.