Only support the new source_span spans in barback.

This releases barback 0.15.0, code_transformers 0.2.0+1, observe 0.11.0+2,
polymer 0.12.0+3, and smoke 0.2.0+3.

BUG=19930
R=blois@google.com, rnystrom@google.com, sigmund@google.com

Review URL: https://codereview.chromium.org//430973002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@38762 260f80e4-7a28-3924-810f-c04153c831b5