dart2js: add --preserve-uris flag.

R=blois@google.com, johnniwinther@google.com, nweiz@google.com

Committed: https://code.google.com/p/dart/source/detail?r=41205
Reverted: https://code.google.com/p/dart/source/detail?r=41206

Review URL: https://codereview.chromium.org//582753002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41244 260f80e4-7a28-3924-810f-c04153c831b5