add a comment to the generated lock file  Review URL: https://codereview.chromium.org//12133002
add a comment to the generated lock file 
Review URL: https://codereview.chromium.org//12133002

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@17951 260f80e4-7a28-3924-810f-c04153c831b5

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