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- Add a new VM object called a Namespace. - A Namespace is a library, and two list of names to hide/show. - Convert Library and LibraryPrefix to contain Namespace objects instead of Libraries in their import list. Review URL: https://codereview.chromium.org//10967052 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@12804 260f80e4-7a28-3924-810f-c04153c831b5