Widen the barback constraints for code_transformers and smoke.
Widen the barback constraints for code_transformers and smoke.

Barback 0.12.0 doesn't introduce any transformer-level backwards incompatibilities.

R=blois@google.com

Review URL: https://codereview.chromium.org//214033002

git-svn-id: 03ce01c0aef5575341110df69fddd5703b005ee4@34462 260f80e4-7a28-3924-810f-c04153c831b5

dart-archive/code_transformers,dart-lang/code_transformers