Make observable transform a barback transform.

R=jmesserly@google.com, nweiz@google.com, rnystrom@google.com

Review URL: https://codereview.chromium.org//22396004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/source_maps@25985 260f80e4-7a28-3924-810f-c04153c831b5
6 files changed
tree: 73192e2bbf6c678d612e5eb6cb4f7c973c0fe89c
  1. pkgs/