Only run [declareOutputs] once for each asset/transformer pair.
Only run [declareOutputs] once for each asset/transformer pair.

BUG=16442
R=rnystrom@google.com

Review URL: https://codereview.chromium.org//227123007

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@34892 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/pub,computmaxer/pub,maxwellpeterson-wf/pub,cjkao/dart-pub,dart-lang/pub