"pub get" doesn't choke on previously-cached unknown transformers.

BUG=https://code.google.com/p/dart/issues/detail?id=21298
R=alanknight@google.com

Review URL: https://codereview.chromium.org//651993006

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41087 260f80e4-7a28-3924-810f-c04153c831b5
Fix web_components test in FF

Review URL: https://codereview.chromium.org//647393003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41088 260f80e4-7a28-3924-810f-c04153c831b5