This CL does several things:
* Adds explicit infrastructure for package sources.
* Adds the Dart SDK as such a source.
* Adds an explicit class for managing the packages/ directory (AppCache, as
opposed to SystemCache).
* Adds an "install" command that installs to packages/, backed up by the system
cache.
Review URL: https://chromiumcodereview.appspot.com//10340005
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@7398 260f80e4-7a28-3924-810f-c04153c831b5