Deal better with missing sources in code_transformers and polymer
Deal better with missing sources in code_transformers and polymer

R=blois@google.com

Review URL: https://codereview.chromium.org//233673003

git-svn-id: 03ce01c0aef5575341110df69fddd5703b005ee4@34978 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/code_transformers,dart-archive/code_transformers